If it must be new , Shimano (2303, $75), Sugino, Velo-Orange, and others still make square-taper triple cranks. There are lots and lots of good used Sugino and Shimano triples that would do the job, though.

As Bob pointed out, the big difference for 9/10-speed cranksets is that the rings are slightly closer together. You can buy thin spacers that spread them back apart for use with wider chains.

EDIT: You should be able to keep the front derailleur -- I used a regular "road" FD with the triple in my conversion, and it worked just fine with the granny ring.