I went to a closing gunstore nearby and bought among other items two Walther airguns in "kit" format; LP3 and LGR.
Kit form meaning that they had been disassembled at some point and were waiting for new seals that never came.
Oddly LGR had extra piston and extra chamber end plug (the part where loading lever pivots)

Seals on both were obviously bad. Tips on places from where to order replacement seals are welcome.
I have 3D printer and TPU material. Have you tried if it works as valve seal material?
Also have a small lathe so I can make small parts.

Valve on LP3 confuses me. I searched for info on how to assemble the LP3 and found some forum posting that had all parts laid on table. Also note that there were at least two versions of valve.
Attached is photo of my LP3 valve body and parts I have for the valve assembly. Is there something missing?

Cannot post attachments so put photos in dropbox.
https://www.dropbox.com/scl/fi/a20xi...o6dz7tap4&dl=0
https://www.dropbox.com/scl/fi/sfshc...e2iq9njxr&dl=0
https://www.dropbox.com/scl/fi/llnqh...zh7wc8a57&dl=0
https://www.dropbox.com/scl/fi/qn3wn...1uhme7t2m&dl=0