Most cost effective source for residential wire and conduit?

120V 20 amp circuit 500' yes run #10. Why just 2 conductors? No ground? Really need 3 #10 wires. Don't pull 10/2 romex into 500' of conduit. Use #10 THHN. Stranded would be easier.
On this kind of stuff Lowes and Home Depot are usually just as cheap as the electrical supply houses.