H.F. is a standard place to buy one, I think it would cost allmost as much to build one unless you con get stock real cheap.
also some flat denders DO NOT require any press at all
see the thread that SKShunter has made of his "poor mans jig"
This is the press i am talking about. it is 12 ton. the top piece is bolted to the side c-channels with 4 long metric bolts. I put the washers between the c-channel and the top piece. It is real solid now..