You don't need (or want) felt if you're glueing it down. Modern construction adhesives designed for flooring cure to a very rubbery consistency. They don't lose adherence as the floor expands and/or contracts, and they make it quiet to walk on. I wish I'd slathered the stuff on when I built my subfloor (and used screws instead of nails), because I've got a few squeaks where I didn't. Glueing the bamboo down with Bostich's Best glue sure quieted things up a lot. I nailed it in some strategic spots to keep it in place while the glue cured between our sessions of floor-laying. Be aware that, at least with the Bostich's Best, floor glue is the snottiest, stickiest, gooeyest stuff I've ever seen, and it is like a magnet for things it's not supposed to get on. Plus, NOTHING removes it once it's cured. Don't ask me how I know this.
As for the leveling, it is unbelieveable how a perfectly flat and level subfloor really isn't either. Minor imperfections in it manifest themselves as major imperfections once the flooring goes down. Don't ask me how I know this.
I think we were able to get our bamboo (which is wonderful) for around $1.85/sq. ft. from a wholesaler in Highland, Illinois. They even delivered to our road (but not to our house - you've never seen a Honda Element so loaded with Bamboo!) for free, because it was almost on their normal truck route. If you shop around, use The Internets and make enough calls, you can find good stuff very reasonably priced. And yes, get samples from as many vendors as possible before you commit...
John