eBay and Craigslist finds

I don't think you'd be the first person to see 'Merckx' and think you've found a bargain, there is a market for frames like this.

If you're after something cheap then do what I do and set up saved searches for 'include description' (531, 531c, 653, 753, 631, 853, 953) and also (slx, sl, tsx, columbus) etc etc. Sometimes you just have to be lucky though (put stuff in brackets like that, separated by commas and it'll search for all of them).
I had a huge drought recently where I didn't find anything but for sub £80 I've had in the past - an 853 frame, a handbuilt SLX frame, multiple 531c, 653 and 753 frames, a couple of Bob Jacksons, and other ridiculously badly listed awesome frames.

Just keep an eye out, sometimes things slip under the radar and you spot it!

I've had to really control my spending on the bay. I think my missus is starting to suspect something when every second week I'm coming home with a new frame/bike. My excuse of storing them at friends who are then returning them is starting to wear thin ;)