@SkipdiverJohn might be able to suggest a better source...?
A friend's hobby is doing-up skip-bikes. He generally won't spend more than about 20 squid on any one. He has picked up some corkers, including a Dave Yates and a Major Nichols.
The obvious place, as my username might suggest - is go look in some skips!. I never pass one by without taking a peek, and I've had some useful gear out of them over the years. Not just old bikes either.
The next places are on the street. it's not unusual round my way for bikes with mechanical faults or some rider abuse damage to just get left out on the pavement. I'm only competing with the scrap man and if I like what I see, I'll have it. If it's up against someone's garden wall, I'll check with the occupant it isn't theirs first though, in case a kid has crash-parked it outside then gone into the house like they do.
It's against my religion to pay anything more than a nominal sum for a fixer-upper or parts donor, and I have never paid £80 for any used bike. My limit is £50 for anything with a 531 frame or about half that for gas pipe steel.