Note that head tubes also need to be cut off perfectly squared. The easiest & best way to do that by far is in a metal lathe.
To that end (assuming you don't have a metal lathe) srdavo's suggestion of just cutting one off an existing frame is not a bad idea at all. It is usually possible to grind & sand off all the extra metal and get something that looks basically new, if that's what you want. All the cheaper bicycles I've seen appeared to be welded, making them easy to clean up.
I was planning another bicycle project once (that never really got started, by the way) and wanted a number of various bicycle parts--gears, cranks, rear 26" wheel, key frame pieces to weld a frame from, brakes, ect ect.... After considering a few options I went and bought an entire steel-framed 15-speed non-suspended MTB from Wal-Mart for $79. There was no way I could possibly buy [separately] all the parts I wanted for that much.
It was a pretty cheap bike top to bottom, but I didn't have any source of free used bikes (-didn't want to go out hunting on trash day, or wrench apart a bunch of rusted busted bikes looking for one good part-) and you can get a project rolling on those cheap parts. If you want to upgrade later, better parts would fit in the same place as the cheap parts you took out.
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There are places some have noted to buy "bike-frame specific" tubing, like Columbus, Reynolds, Tru Temper, ect. The main difference between this tubing and standard structural tubing is that the bicycle-frame tubing is double-butted in lengths specific for building conventional (upright) bike frames from, and the frame lugs and dropouts commonly sold are the right size to accept it. Industrial tubing is pretty much always the same wall thickness along its entire length.
