I've bought a couple osage staves off of ****. They were better in person than they looked in the pictures. They are still drying though, haven't worked them yet.
From my limited experience, it seems that the vendors selling staves know which ones are good for bows and price them as such. You could find one that fits your criteria and post a link here. I'm sure the experienced self bow guys can tell you if it's a good buy.