Do you mean the piston seal, or the washer at the base of the spring?
Airarmsspares.com is my go-to 👍🏼 Should be about £12 plus post.
All mks of TX and PS use the same seal, and probably also washers if it is a guide washer or something you need.
If it’s been running without a piston seal, inspect the piston head and inside comp tube for damage 👍🏼