suspensionforkparts.net stock elastomers for several shocks, I've heard some people use a certain type of Landrover Bushing as the elastomer for the rear shock, if you search on here you should find some info on it.
Go visit the Proflex/idriders forum, lots of Proflex knowledge on there.
If it's for a Peugeot though it may be some crazy size, have you any pics? it may be worth looking at suspension fork elastomers too, but you'll have to take some measurements.