Vacuum delay valve - hard to find!?

[C10wip]

I had read that the vacuum delay valves sometimes get clogged over time, and that replacing them wasn't a bad idea. Looks like a cheap plastic part so I thought I'd do that. But... these things seem to be hard to get a hold of.
The part numbers I found were BWD EC312, Standard DSV20, Echlin CRB 2926, and AC Delco 214-580. None of the chain auto parts stores stock them and NAPA couldn't even order their version. RockAuto has them in their listings, but when I ordered from they called back and cancelled the order a day later, same deal with mypartsgarage . So, I found one on eBay and ordered it; we'll see if he really has it.
So my question is - why is it that no one stocks these? It doesn't look like they have been superseded; is it that they never fail so no one buys them?