Just get a normal PSU and buy molex splitters and cut the 12v wire off the split ends (leave the 5v safely attached).
Be sure to check the actual rating on the rails. If going with an ATX PSU you're probably better off using the PCIe or CPU connectors which typically have a dedicated or at least high-powered rail. Assuming this board needs 80w and connecting a disk/peripheral rail off a cheapo 150w PSU could well end in trouble.