I just went through this process, and was only marginally successful in finding a source. I was thinking that Guitarmill and everyone had to be making huge margins on instrument makers who didn't know how to source their lumber.
Well, it turns out not to be the case. The real problem is that the dimensions and amount of wood we use for instrument building is somewhat odd. If you only want to do one, you're probably better off buying a blank. Otherwise you're probably going to have to buy more wood than you need and wind up paying more anyway.
I wound up finding a supplier for swamp ash that is somewhat cheaper than buying a blank, but one face needs to be planed and I had to buy a 6' board. I'll get 4 bodies out of it, but it wasn't a feasible purchase until I had interest in others having a body built for them.
Also keep in mind that there are several different kinds of mahogany. Honduras mahogany is closest to what they used in Les Pauls. African mahogany is cheaper and works, but it looks a little funny (this is what Warmoth uses unles you specify something different).
For body blanks, these appear to be among the better suppliers:
http://www.gilmerwood.com/ http://www.guitarmill.com http://www.usacustomguitars.com Prices are all similar for all of them.
For just plain old lumber, the best I've found is
http://www.curiouswoods.com I've been to their store and dealt with them through email and found them to be incredibly helpful, though they know little about instrument building.
If you're building a tele, you want 8/4 wood, which is 1.75" thick. The total body width is 13", so either you need 8/4x8" and join two pieces, or you need a board at least 13.5" wide to make a 1-piece body. This is the killer, they're kind of rare. The piece of ash I'm getting is scrap from another order.