Good question.
We've compiled an impressive list of online stores for product sourcing. These stores are scanned by Tactical Arbitrage portals.
Click the link below to see the list. The list is not complete. It grows and grows.
In addition Tactical Arbitrage boasts a feature that allows you to (with a little coding know-how), add your own sites to scan over and above the built-in options.
Pretty nifty, hey?