It seem that many make their own GC using editing software. (me included)
However there are several places you can find them such as massage warehouse, scripts, etc. Anywhere that sells massage supplies.
There are several threads on this in the promotional materials section:
Gift Cards:
http://www.massageplanet.com/forum/viewtopic.php?t=668
Valentine GC (has good info)
http://www.massageplanet.com/forum/viewtopic.php?t=617
Gift Certificates:
http://www.massageplanet.com/forum/viewtopic.php?t=604
Don't forget to make sure your GC meet federal regulations for expirations. Here's a link to a site that provides all the info you'll need. It has a short explination by state, then more details towards the bottom.
http://www.ncsl.org/programs/banking/GiftCardsandCerts.htm
Hope you find this helpful.