Start checking farm auctions and estate sales. Find your local ABANA chapter (Artist Blacksmith Assn of North America), group members may have extras or know where to get 'em. If you're willing and able some (many) bits and pieces can be made (more time than money). Iforgeiron.com has a plueprints section with lots of plans.
I made my forge from an old weber bbq grill. (A local guy finds this ironic as according to him the origional grill were converted from unused rivit forges. Ah the cycle of life.) I put a hole in the side for an air pipe (don't use galvanized). Consider the pipe expendable unless you want to go to the trouble of making it water jacketed. I lined the insides with wood ash (many possibilities for lining material), leaving a depression for the fire pot. I set it up so the air pipe sits an inch or 2 above the bottom of the hole and 3 or so from the top. All you need is an air source for a suitable forge.
If you don't mind working from the ground, a fire pit can be dug in the ground and a pipe run into the fire for air makes a trench forge.
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