have you checked out Mr. S Leather? or maybe the stockroom? awesome selection at both sites.
if you can't find the right O-ring size, ya might try looking for models with removable O-rings. then you can swap out to meed ya needs
ETA: in the shop, i usually the big boys to pony up the extra cash for a Spider Gag. if you buy them from a sex shop you'll pay more money. ya best bet is a medical supply house.
make ya own.
two leather strips.
6 snaps.
one O-rings.
lay the straps out flat, same side facing up.
following directions on the snap packaging put two snaps on the far ends of the two straps. one side should have two female bits. the other side two male bits.
the other end of the straps need two sets of snaps on both sides. the arrangement is (starting from the leading edge) male, male, female, female. you want the snappy part of the snap on the front side of ya gag straps.
avoid leather wrapped O-Rings, unless you are partner exclusive, germies and such can get caught in the leather. *blech*