I have used the card packs you can buy at Hobby Lobby or Michael's, but have pretty much stopped using those because lately they all seem to be scored improperly so when folded they don't line up well. But the plus is you get envelopes with the prefolded card bases.
I have now started cutting an 8 1/2 x 11 piece of heavy cardstock in half, folded it makes a card that will will in an A2 size envelope, which I buy at any office supply store.
When mailing, I just have to take my chances that the postal service won't mash or mangle the card inside, lol.
Hope this helps!