Chris,
Sherwood makes the knobs you want. Almost any LDS can order them from Sherwood. (Whether they WILL or not is another question...)
Here is OMS's take on the issue (which is another reason I never purchase OMS gear):
"WHY OMS VALVES DO NOT COME EQUIPPED WITH RUBBER KNOBS
OMS manifold valve knobs are manufactured from medium durometer plastic to prevent wedging and accidental turning should the manifold come in contact with the overhead environment such as those encountered while cave diving or wreck diving.
Rubber has many drawbacks. It is subject to rapid UV deterioration and sudden failure, is usually a low durometer material, has a high coefficient of friction (it s grabby), and is therefore more prone to being turned by anything it comes in contact with. For example on a set of manifold doubles, the Divers left side valve knob (rubber) would wedge and turn to the closed position (unexpectedly shutting off the gas supply) more so than plastic. On similar note Cave divers discarded rubber cylinder boots long ago, after they discovered that they had a tendency to wedge in restrictions, causing the diver to become stuck."
Sherwood knobs fit nearly all major manufacturers manifolds and valves. You can find them listed at numerous online dive shops, such as this one (second item from the bottom):
http://www.divingniknaks.com/products.php?pg=2
Suggest you either do a search on this board for Sherwood Rubber Knobs or try googling the same keywords. I found something like 11 pages of hits, I'm sure one of them is convenient to you with prices in $$.
