Dig around online and you can find instructions on making piston seals from plain leather.
It can be as simple as soaking the leather, and pressing it into a proper sized hole in a block of wood, or a bit more complicated if the design calls for it.
Gone? Gone-gone, or fubar-gone but still there? If it is still there, you can often resurect it by carefully working it out of the socket it is in and, for lack of a better term, fluffing it up, loosening up the packed down leather so that it will form a seal again. Or you can add a shim layer below the seal to make it work.
If it is indeed vamoosed-gone, then you want to measure the socket and get on the phone with D&L Airguns to see what they can do for you. You want to know the inner and outer diameters, as well as the length, in mm's. Got a CTire digital caliper yet?

Heres the excuse you needed!
They can be made, but if you don't have a metal lathe (maybe a wood lathe) to do it on, it'll be a PITA. Basic process was to cut holes the correct size in a stack of washers cut from some thick, hard leather, mount them on a mandrel so they are compressed tightly, and turn the OD to the size you need. Try them for fit, and make adjustments to the size of the next batch until you get a working part.
In my experience, you can stuff a LOT more leather washer material into that space, than you would think you could.

I used a flat punch and hammer to push the seal into place with.
Tandy in Calgary is a good source for thick hard leather scraps suaitable, as well as the thinner material for the piston seal.
Cheers
Trev