Tuff Pex Fittings

[Max1]

I am looking for the yellow rubbers that are in the 1/2" black female pex fitting's. I have checked local hardware stores as well as the pex tube manufacturer as well as pexsupply.com to no avail being told it may be a special rubber for the application or to that effect but they seem to be in all the above mentioned fittings and seem fragile and one some what worn were the city water connects to the pex.
I would like to have a few spares anyway just in case and it bugs me when I cant find something which would seem to be a common item
Thanks

[Bruce and Jenna]

If you can't find 'em, call Peterson. Vickie will probably slip a few in an envelope and mail 'em out.