I never checked Lowes, but it is definitely a regular stocking item at Home Depot. That's where I would get them. Can't beat $2.50 for 10 of them.
Edit, the Lowes variant. Also appears to be a regular stocking item.
I don't know what to tell you. Our Lowes had a wall of O rings with the largest being a #36. They did not have any Danco rings at all. I asked the clerk and they did not stock any larger.
Home Depot is about 3 hours away.