I haven't heard of any 10 rd mags. Up in Canada we are basically in the same boat as you except that we are limited to 5 rds. I don't know if this will pass scrutiny in your state but what we do up here is this;
-find out were the follower sits with the rounds loaded, drill a hole in the spine bellow the follower and insert a pop rivet, or;
-attach a piece of rod or strapping to the bottom of the follower (use epoxy for plastic ones, brazing on metal followers) that is the correct length to only allow the number of rounds desired to be loaded. Just to extra anal, you could then insert a small pop rivet into the floorplate so that it could no longer be removed, or;
-find some cheap uzi mags and shorten the mag body and spring down to the proper length. This last method is obviously the most difficult as it requires a lot of trial and error to find the proper measurements but it is obviously the most permanent. The other methods, while permanent, are easily changed back to normal capacity with a drill and cut off tool.
I hope that this helps.