i have used the blue poly hydrolic ram seals , available at hydrolics suppliers , with excelent results , for uncommon sizes convert back to imperial , these have a better memory than the nitro ones , i have where necessary machined out the seal holder or sleived it down or whatever , i even have a set in some betor piggy backs with pitted shafts and they are holding some pressure !!, nb, i haven,t got any gas in the ones i have done , a good cheap cheap alternative i can buy over the counter for basic type shocks , on the list to try also and available in a simliar material is gas strut seals