A few bike specific companies sell replacement plungers. The hard part is finding the dimensions of them but probably something will work. Check websites for spares; Silca, Topeak, Lezyne, SKS, Zefal, and probably a few others. Scroll through this place for some examples. A few listed (and a lot of stuff that doesn't apply).
Bicycle Pump Accessories and Small Parts - Modern Bike Also pneumatic or hydraulic pump suppliers probably have something. Hell, my mini grease gun uses the same type of washer, just not sure of the size.