I'd add KEH, Adorama and maybe B&H although I had to return an M2 (rated 9) that I got from B&H due to lens mount distortion. Also Popflash might have one for sale time to time.
Typically, buying from known resellers will guarantee you the cosmetic and basic operation condition that they state, but there might be hidden problems because they test each camera less thoroughly (with an exception of popflash which I've heard only good things when it comes to used Leica condition) compared to Sherry K or GOOD RFF members. Of course you can return it or have it fixed if you buy from reputable resellers.
I'd personally go with RFF classifieds. I trust RFF members over evilbay or large scale resellers. Also, having resent CLA is a key for M2 or any other old cameras. Unless it's been done and proven, you probably wanna add some cost for CLA to the budget just in case.
There are at least a couple of nice M2s (one user, one ex+) in RFF classifieds as we speak. (I'm tempted, too!)
