Rated ones? I usually get mine from the usual speleo-vendors - Inner Mountain Outfitters, Karst Sports, etc. Not really cheap, but there aren't too many stores that stock them in the US.
If you don't need life safety rated ones you could always try your local hardware store.
Camp sells an 8mm stainless QL for $8 that is rated for 40kN. http://www.camp-usa.com/products/carabiners/oval-quick-link-934.asp I haven't used any of these so I'm not sure if the gate opening is enough for 11mm rope. Another website claims the gate opening is only 8-9mm. This is significantly cheaper than the Maillon Rapide 7mm oval that goes for $14 from the usual vendors. I have used these 316 SS QLs: http://www.uscargocontrol.com/Rigging-Supplies-Hardware/Small-Quick-Link-Stainless-Steel-316/Small-Quick-Link-SS-T316-2-1-4-Length They have a WLL of 2,400 lbs, which is roughly the same 40kN failure strength, and they are only ~$4 with shipping. They have a gate opening of 3/8" (9.6mm), but you can get an 11mm rope through it. They also sell a long oval version, which will have a larger gate opening.