Price one... its not cheap...
Oops, Brian was typing faster than I was... but I'll still post my thoughts...
But I bought one to have on hand, as I did when I was servicing C10's, and if you buy one, you will find that you will only need it if you totally damage the one in place...
These are pretty tough gaskets, and truthfully I feel they last a lonnnnnng time, and can be cleaned, and reinstalled many many times over, if you are careful,
And clean it well to remove the oily coating, and sealant /adhesive used at the factory to hold it to the cover, and also completely clean the groove.in the cover to remove all of that adhesive, and also clean all of the sealant used on the semi circle cutouts and anything on the head, you can reuse the gasket without issues... I use acetone for the cleaning, and stout blue cleaning wipes.
Before I install, I run a very SMALL (about pencil lead diameter) bead of black rtv around the center of the groove in the cover, about a hlaf hour before I plan to reinstall the cover, and then stick the cleaned gasket firmly and evenly in that groove, and set it upside down on my bench with about 5# of weight on it to "glue" the gasket in place for handling... I used to use monkey grip tire patch glue applied with a large syringe for that, but cleaning the syringe was a p.I.t.a., it did work great tho for gluing the gasket into the groove, and basically recreates what is done at the factory.. apply the glue, insert the gasket, then pull it back off and let it skin up for a minute or three, then stick it back in... very effective.
I then "smear" a thin layer of rtv around each semicircle area, I don't goober it up with a bead, that is wayy overkill, and it smooshes out into the valve area.. not good.
Re install the cover, while paying close attention the gasket didn't dislodge, and is evenly seen around the perimeter, and tighten it snuggly.
I've never yet come across a cover gasket that "hardened" to a point it was no longer pliable... but I'm sure someday I will... that's why I have a spare...