i don't think oakley sells any directly. some shops though that offer oakley customization would sell you just a strap. there's a shop where i ski (darkside snowboards @ killington, vt) that does this. you can buy separate frames, straps, lenses, and rigs, or put them together and build them custom. if you know any such shops check them out.
or just go to sell/trade and trade/buy just a strap.