I don't know about GB right now, but I think J&R might be a good option. I haven't been there for a while ( at least a year ) but they have a pretty good latin music selection.
There's that shop that reopened in the Times Square subway station, some good stuff, but they are pricey.
Casa Latina is limited -- they tend to have mostly new releases and everything is kept behind the counter. I'll go there now and then when I want something brand new hot off the press, but I always call ahead.
If I think of something else I'll let you know.
Good luck!