Can we not purchase these outside of the core set?

No, there's (currently) no source for damage decks except the core set.

If you've lost or damaged a card, though, you might email FFG customer service and ask them to replace it. They're usually very good about that sort of thing.

Alternatively, wait exactly 1 month for the new EP VII core sets that will have a new damage deck, a new EP VII X-Wing and 2 new EP VII TIEs in it.

There is absolutely nothing official to support the above comment .. just my wager.