For slabs, my go to is Northern Custom Hardwoods, 7377 Weiland Rd, Weatherford, TX 76088
Owner: David Sinkola, 817-501-8752. Specializes in large slabs, generally sourced from the Northeast. I have purchased 3" thick walnut slabs over 50" wide from them as well as large slabs of numerous other species. Call before visiting since they have variable hours.
You might also check with these DMS makers who also have wood for sale:
Kyle Alty (various species) & Jeff Bob Whitcomb (mesquite).