How do you obtain cards from ships you know you won't use? I would love to have the "Gunner" card, but I play rebel and don't need a Slave-1. And C-3PO? There's no chance! There's that guy in Great Britain on eBay, but I hate to go that route, being in America.
I saw the trade thread on here, which is great, but I just don't have good enough stuff to trade. How do you do? Do you just pony up and buy the ships?
I use all the ships, so it's not an issue
I use all the ships, so it's not an issue
I appreciate that, but I don't have the funds to invest in both sides. I like Rebel, so I stick with it.
If you're playing casually, use proxies. If you're playing in a tournament, borrow the cards from a friend. There really isn't a good way to buy the cards individually.
Trading is not a very likely option, the cards you are after, or almost definitely the same cards other players consider must-haves. Even if you can buy cards seperately, shipping costs will add up to where the price difference with a Slave-1 *with* awesome model is small enough for you to be able to justify buying the ship to yourself 
Though, cards almost always are re-released with other ships in a later wave, so maybe the next ship will be to your liking?
And there's always the option of just printing a proxy (if your answer is that you would like to play tournaments, then consider that you can't play in a sportstournament without buying proper gear either).
I understand that getting everything isn't always feasible, but I always get a little confused when someone says "I play Rebels" or "I play Empire". Aren't you even a little bit interested in the other half of the game?
With that out of the way, the options for getting legitimate copies of upgrade cards really are limited to the secondary market or waiting and hoping FFG releases them in a new SKU you're interested in purchasing.
If you're planning on just using the upgrade in friendly play, however--not at tournaments, where this is illegal--I would recommend finding a high-quality image of the card, printing it on high-quality cardstock, and using it just like any other upgrade card.
EDIT: Looks like for Gunner, at least, you won't have to wait long. Fourth upgrade card from the left in the top row:
You could buy the expansion pack, take the cards you want and sell the rest with the caveat that you've taken some cards.
There are lots of players who want 2 Firespray's, but not necessarily 2 of all the upgrade cards in it.
Aren't you even a little bit interested in the other half of the game?
Well in the OP's case it's less that he's not interested and more that he doesn't have the resources to play both.
If you have a max of $200 to spend on X-Wing, would you rather buy $200 worth of one faction giving you lots of options for that faction, or $100 of each? You would have fewer options over all going that route, because half your ships can't be used with the other half.
At the OP, I'll also recommend just using proxies unless you're going to play in an official event. Even then, if you know enough people odds are you can borrow what you need.
Aren't you even a little bit interested in the other half of the game?
Well in the OP's case it's less that he's not interested and more that he doesn't have the resources to play both.
If you have a max of $200 to spend on X-Wing, would you rather buy $200 worth of one faction giving you lots of options for that faction, or $100 of each? You would have fewer options over all going that route, because half your ships can't be used with the other half.
The OP said as much, and I really do get it--I'm lucky to be able to spend as much as I do on this game. It just always strikes me as odd to see people identify with a faction: not "I can only afford to invest in Rebels right now, so I don't have a Firespray" but "I only play Rebels, so I don't need a Firespray". I feel like the reasoning there is subtly different.
Also, it's largely beside the point--but given that hypothetical $200 budget, I personally would still invest in both factions (buying online with free shipping):
At the OP, I'll also recommend just using proxies unless you're going to play in an official event. Even then, if you know enough people odds are you can borrow what you need.
Definitely.
I feel like the reasoning there is subtly different.
I agree, with that. I can't understand why someone would limit themselves to 1 faction if they don't need to. It's not like 40k where you have to spend $500 to put together an army for a single faction, which makes collecting more then one prohibitive for most.
You're right though, having access to both sides does keep things from getting stale, playing a 4 X list is nothing like playing a 4 Tie Interceptor list.
I generally take turn about flying Rebs and imperial. And while I know I'm fortunate to have enough dough to spend on x wing, the reason I do it, is because it means I have a decent knowledge of what my opponent can do, irrespective of the list they bring.
I'd say vorpal sword's list of 'essentials to purchase' is pretty bang on, and shows just how much you can get for a very small investment in the game.
As to the op, I know it doesn't help you much, but the only way I can think of for you to get the card 'on the cheap' is to buy the expansion, and hawk everything else on eBay.
Ya'll got it totally right. My budget is limited. The owner of my local shop is a good friend of mine, and I feel that I should by my stuff from him, even though it's at full MSRP. I gotta support my shop if I want it to stay open. I'm new to the game, and have been trading Magic cards to the shop for ships. Someday, I'll start getting Imperial ships, and those builds look like a lot of fun. But it's just not in the cards right now (pun not intended, but I kinda wish it was).
I like the ideas of proxies, and will try them out. I'll also ask around about borrowing cards for tournaments. I'm starting to make some friends in the game.
Thanks, as always, for your input. This forum has been nothing but helpful.
Another idea on the proxy side. If you play at the store and don't want to have printed cards (so your friend/owner gets sales and you don't look like your on a budget) use one of the squad building programs and print your squad out and have just your ship/damage cards with you.