In repairing a TD filler, do you replace the small rubber washer (the one that goes around the screw that secures the sac protector to the blind cap) or do you just use some rubber cement? Seeking pros and cons of both options, additional options, and a source for the rubber washers.

TIA, southpaw

are these useful?: SNORKEL PFM Seals Parts Gaskets Washers

I usually don't. I dab a bit of my section sealant on the old washer and that gives me a very good seal.
All you need for Touchdown repairs can be found at very competitive prices in the Tryphon catalog, no need to use EBay!

Tryphon Catalog - all you need is here!

I regularly receive inquiries about the need for (and availability of) these between-blind-cap-and-plunger-tube washers. In my experience, they virtually never need replacement, nor any additional sealant (though a touch of silicone grease wouldn't hurt, and would prolong their life by excluding ozone). And in those rare cases where one is found cracked or torn, or ends up dropped and lost, a new one can be easily punched out of 1/32" rubber sheet (and the cutting can be sloppy indeed).

As for other Snorkel, Touchdown, and plunger-filler repair supplies, please also take a look here, and at the linked repair instructions and articles.