Best bet is still Falfiles, but you can also try Gunbroker, or your local state forums, like CalGuns if you're in CA.
As to what a "decent" one is, you'll get answers that are all over the place.
It can go from the really nice, unused STG58's, to the beat to hell R1's and Rhodesians (which are my personal preference).
If you want to do an Israeli build, find out how close you actually want it to be. Do you just want izzy features, or do you really want an old izzy kit? If you just want a FAL rifle, you can build all sorts of features into it - won't be "correct" period or model stuff, but if it's a good shooter and fits you well, what the hell.
I'd wait until you get on the 'files and start seeing what's out there before you jump the gun. Also, it'll probably be cheaper to just buy an already built rifle than it would be to build your own. Or have it built.