the 88 has been around for a long time and they sold for over 300 bucks in their life span
the idea of not fixing it is absurd
parts and parts diagrams
see that Umarex has their name associated is not good they would want you to send it to them
https://en.wikipedia.org/wiki/Walther_CP_88 i would replace the whole valve but they are hard to find but Protek shows one part 416.60
http://www.proteksupplies.co.uk/walther-spares.html now you are more then likely going to have to go to the UK for parts but Chambers has stopped shipping to the states for now, Knipp has not said anything as of yet nor has Protek
now any Orings you would find in the valve would have been Urethane and the replacements need to be the same there are some kits out there just do some searching
this is Knibbs parts diagram and their prices with VAT on them so the price in your basket will be less if it know that they would be shipped to the states Vat i believe is 19%
https://www.airgunspares.com/gunspares/airgun-spares/walther/cp88.html