Are there any available that don't swell or fall apart like the normal parts store garbage does? I have to replace mine occasionally as the material gets soft and won't hold the breathers in anymore.

I bought the last ones from Summit, I think they were their brand. The thickness of the valve cover has to match the grommet, mine would split at the groove due to mismatch.

The ones from Moroso have always done very well for me. Haven't used any since I switched to cast covers with bolt-on breathers when I added the Pan Evacs a few years ago, but we put some on my son's Chevelle about 2 years ago, and they are still holding the breathers firmly.

Must be your oil. ???
I've got B&B and Moroso baffled style grommets that have been good for literally 20 years....?