If your going to buy new I recommend:
BMC Acrylic, there a sponsor here and for a 34x34x10 with 4 frag caddies for 210. Let Ken know Brandon refered you from Coral Seasation.
You can also go the ebay route. Walmart tubs or hydro tubs. 40b or 33/20 longs.
Used:
Someone will probably shoot you a pm here soon
If you go to forum, then coral and prop, you will see a 35 page thread where everyone shows off thier different systems. Should give you a great idea of what can be called a frag tank in these endevours.
Brandon