Are we talking testing, demonstration or production?
If this is just for testing gameplay I suggest you go for an even cheaper solution than Mark suggested. Assuming you some other cards in that size then just print yours on normal paper and sleeve them together with the real playing cards. At this stage all you need is a cheep way to test out potentially numerous revisions.
EDIT: I recently discovered a card game by the name of "Summoner Wars". It is self published by the designer (Colby Dauch) using some card printing service (in China I think). You could ask him what service he uses or could recommend. You know, from designer to designer...