John Knibbs is probably the nearest supplier to you.
You need to ask for a 25mm Weihrauch HW77 piston seal, best to measure what you get as his grandson served me last time and when I got home I found he'd given me a 26mm seal.
I take it you know that the weihrauch seal will need an adaptor to fit the TXPiston ?
Think they're about £11.
Edit:-
Any problems with Knibbs and I'll let you have one.
All the best Mick