They are available from numerous sources. I purchase most of Weihrauch seals from Vortek.
I may be wrong, but I do believe the .25 Weihrauchs use a plain oring for the breech seal. Pry it out with a pin and you should be able to measure it, and order a sack of 100 from McMaste Carr for a few bucks. Or the Oringstore if it's oddball.