Pro sono is run by a guy called Philippe in fourways. He is one of the worlds leading sources of african blackwood, an incredibly dense and heavy rosewood grown almost exclusively in namibia.
He also has some pink ivory and other local woods, as well as some palisandre, which is very similar to indian rosewood.
I have bought acoustic back and side sets, as well as a few african blackwood fingerboards from Pro-sono. nice woods, all of them. I have not built with the back and side sets yet, they are still just chilling in the humidity cupboard.
As for the piece of wood you are looking for, none of those supplies is going to have pre-selected electric guitar bodies. You arte going to need to determine the size of the plank you need and approach them for a piece that size. Your best bet would be to get them to cut you two bookmatched blocks from the same plank. That is to say, if they have a block that is 20cm thick, you slice it down the middle so that each one is 10cm thick. and then join them at the centre, like an open page of a book.
For solid body guitars, you will be using quite a hefty chunk, so you're unlikely to suffer from the issues usually associated with flat sawn tone woods, however, I would still look for something quarter sawn, for strength, and stability when drying out.
This image should show you what I mean:
Finally, as for the drying of the wood, the only way I know to be certain is to keep the wood on hand yourself for long enough. Keep in mind that a big, thick plank like that needed for an electric guitar will take much longer to dry out than the thin back and sides of an acoustic...but would also be prone to less warping as it dries.
