Most likely ebay if you really want to fix it may cost between $25-40 for a new gas spring not counting shipping + tax. However if you want something better I highly suggest one with a coil spring this way you don’t have to worry about leaking if you don’t shoot for a long period of time.

Check out the Ruger Blackhawk this is the Chinese clone of a Diana 34 minus quality, the T06 trigger, and the classic Diana dovetail rail…just has conventional dovetail with a scope stop. This also can be modified with an upgraded spring.