I, too, print on 13x19 and trim down to 12x12. I have a "Quartet" paper trimmer that I bought at Staples. I don't remember exactly how much I paid for it, but I'm sure it was less than $50. I hope that helps.
Edited: I couldn't find it at Staples online, but I did find it at OfficeNation (somewhere I've never shopped but got good reviews):
http://www.officenation.com/webapp/o...fVend=pgrabber