With the current strength of the pound against the euro, Madinter in Spain is a good source, but postage is still expensive and you will probably need to buy bigger bits than you need. I buy their B grade guitar fretboards for use on ukes, but I order enough to make the postage worthwhile. You can use the spare wood from guitar sizes to make bridges, headstock veneers, neck blocks etc. A bass guitar fingerboard will easily yield two ukes. The other option is simply to spend some time searching eBay - plenty of Sapele billets around, but look carefully at the grain pattern. Sherwingroup currently have some quarter sawn straight grained pieces. If it is more than you need, you can re-sell the excess.