I think I remember there being several past threads which suggested that Adams was the supplier that Prompt used.
I seem to recall somebody saying that the replacement OEM suction cup was less than $2 a piece when he bought them from Adams.
When I go to their website, they have all kinds of suction cups (go HERE
) so I'm not sure which one exactly matches the one you need.
I'm thinking it's one of these ...go HERE
However, those are all priced by the bulk. Maybe you can call them to see if they'd sell them to you in smaller quantities and help you find the exact one you need ...give them the application (Prompt sunshade) and the size (dimensions) of the suction cup you're looking for.
800-237-8287 option 2
ETA: Actually, do you think it may be THIS ONE
? (2 3/8" diameter), it's only $18.55 for 50 (plus shipping) so less that 50 cents a piece and you'll have a few extra when you need them.