Have you got a big box store within reasonable distance ?
Lowes seem to be better than HD, for some replacement
items such as you are looking for. Sears Hardware stores
are also on my list of better stocked replacement plumbing
parts places to try. First off, try Googling American Std
Replacement Parts. You may be surprised. Most plumbers
use plumbing specific outlets, most of which specialize by
carrying one or two specific brand names. You may come
up with a few hits that you can phone or E-mail to begin
your search.
Good hunting
Uncle Bob