scrap yard, walk away with 5 quality frames and various pieces... all it comes down to is knowing quality, reading labels if any, and yeah...taking the time to scavenge...or forage, depending on personality
pay 20 for a good frame usually, tops. they dont care what they are, its all just scrap to them...
avoid anything with caliper brakes. especially stamped steel ones. if theres a stack like there is at my local yard...theres no reason you would look at a caliper brake setup other than to figure out how to get that 5 speed hub with disc brake out from under it...
shogun, GT, giant, trek, cannondale, yarda yarda ad nauseum. theres more good names than bad that a list wont ever cover it.
cro-mo was mentioned.
if the sticker says 4130 or cro-mo....GRAB IT.
just be aware that its better to braze cro-mo unless you got the right filler rods and can oxy/tig weld... its an alloy steel, and does need correct welding procedures, as well as post weld treatment.
hi ten will do, but avoid any really cheap n nasty frames.... but on that token...usually the cheap frames are great, especially if you plan on chopping anything...its all the attachments that fail...you can find upgrade parts at the scrapyard too
brake levers, alloy cranksets, you name it, theyve got it...