I found a bunch of sources at one point searching u channel rubber extrusions.  Problem is the quantity usually has to be quite large to order from a manufacturer.  I'd see if one of the manufacturers would be willing to sell you scraps.  Some offer samples of their products, but asking for them for your intended purpose would be a bit disingenuous IMO.  If I were doing a helmet mod right now, I'd just use bedliner.

Try and auto supply store, ask for door edge molding.

If not, try McMaster-Carr - rubber only



Rubber with metal core insert



which looks like the stuff we use on our fiberglass fire helmets. Looks like they're available in shorter lengths.

The rubber is available in 10' lengths as is the metal core version.