I don't know of anyone on line for ash lumber but you might try Newfound woodworks. Michael might be able to help you. Another possibility would be black walnut which as you might guess is very dark in color but it bends better than ash and is very attractive. Try to find a saw mill as opposed to a lumberyard. You will more often find what you want from them and it will usually be green or air dried which bends better than kiln dried.

Kent