I don't know if they're a national chain, but here in New England there's a chain called Woodworker's Warehouse (I believe they're online too) that sells mostly woodworking specialty tools - but they always have bins of many different woods in all sorts of sizes, at fairly reasonable prices.
I bought a 12" X 12" block of burl Claro Walnut there for $25 a few years ago, that I've been slabbing knife & handgun grips off of, since.