Is there anywhere out there that I can buy individual upgrade cards? I currently have need of a single card or two and I don't want to buy more ships that I'll never use just to get the cards (I hear this was a huge issue for xwing too)

Ebay is about the best option you'll find to buy a given single card.

I was worried about that might as well buy another Nebulon B as the X17 on ebay is like 13$ canadian

That was my fear. I need two more X17 cards. I am not buying 2 Neb Bs.

That was my fear. I need two more X17 cards. I am not buying 2 Neb Bs.

Why don't you buy two new, take the cards then sell the bits separately. Sure you'll come out even.

I got luck early on and found a full set of core non uniques for about 5 quid on ebay

Also, if you are just playing for fun most people aren't going to have an issue with proxie cards.