I got my o-ring at a Jax store (a local ranch supply, hardware, outdoors, etc. store - great place!)

They didn't have any #61 o-rings. But I thought ahead and had taken my Victory screw with me, so I just tried various o-ring sizes that they did have (which was tons of them) and found a perfect fit. For all I know, it WAS a #61, just not labeled that way.

Ace Hardware is the go-to place around here to find whatever you need. That, and McGuckins Hardware in Boulder too.