Do any of you guys ever see the fill valve seals for the Fluidmaster 400 ballcock. I can't find them in my area and I need a half dozen or so.

That's a 400A, right? Amazon.com: Toilet Ballcock Top Seal - Fluidmaster #400A: Home Improvement

No it is a Model 400, very popular back in the late 50's and the 60's and 70's, the top was stainless and they lasted forever.

I think the seal is a #524

Fluidmaster 400 A ? We don't replace seals. The entire valve costs about $7.00. Takes 2 minutes to install. It is so cheap that it doesn't make sense to replace only seals.

It is not a 400A but I did have the same thought. They are quick, cheap ad easy to swap out with a 400A.