You mean like the ones that come with a SF M1 IR light? Maybe I'm not getting your search right. If your talking about using a square O-ring to fill in the recessed area on a tailcap like the A2 and such I like to use the square O-rings commonly used in the paintball hobby world. Problem is the widest I've found is 3/32 so you need a couple to do the job. But it does look nice once installed.
You might want to send PM to folks who bought them. I only picked up 4 (all in use), and my name was spelled wrong on the list, but others may have spares. I'd suggest starting with whoever picked up the most.