I managed to get a BA #36 from a boardie, but haven't seen one in the wild. I'm also missing WW #95, GL #70, and Superman Annual #8. I've got the Steel #7, and honestly can't remember anything specific about how I came across it.
Everyone's mileage may vary a bit in terms of some of the common books, but for me Green Lantern #52 and Superboy #4-5 are so common I have stopped buying them. Same with Adventures of Superman #508 (I must have 3-4 doubles of this at the moment). Superboy #1 is pretty common, Steel #1 is pretty common, as is the Supergirl #1. Batman #507 and 509 are pretty common. And I agree, Robin #8 is pretty common and so is #9. Flash #92 can be gotten off of eBay any day of the week if you want to pay the price. Really any of the Terminal Velocity Flash issues are reasonably common, save the #100 for some reason (I have only found one in the wild and it was hacked). Power of Shazam #19 is fairly common.
Certain collections are kind of funny that way. Like the Legacy books posted above, for example. The Catwoman is very difficult, but the Batman issues aren't that difficult. The Robin issues are less common, and the Detective issues are also less common in my experience. Seeing as how they were released as box sets, why are some issues from those sets not too bad while others are really hard?
On the flip side, I have only found one Batman Adventures DCU in the wild. No DC vs. Marvel found in the wild. The 1996 are generally tougher, though there are some exceptions (at least in my area). I finally got a Sovereign Seven #1, but that one is not easy.