Tuning Gamo Hunter 880 parts?

Howdy from East Texas.

I have an old Gamo Hunter 880 that’s way, way overdue for a rebuild. Problem is, I’m having a hard time finding parts to make it happen. I called Gamo USA, and of course, they don’t support that gun any more. If I’m not mistaken, it has a 25mm piston, but I have no clue on spring size or breech oring size. This gun used to be very accurate, but now shoots like crap. It seems pretty decent quality, and I can’t detect any barrel droop or slop. Any ideas where I might locate what I need to get the old girl back in shape?

TM
 
Thanks Enkey,

Unfortunately they seem to be out of stock on most of the parts I need, and don’t have o-rings listed at all. I guess I’ll keep the old gal retired until I find what’s needed. Or hang it up as a display in my man cave. It was my first serious air rifle and took out hundreds of rats from my grampa’s barn, and squirrels from my and my dad’s yards. I really wish Gamo would still support their products the way Weihrauch and Air Arms do, but they’ve moved on to plastic, somewhat disposable guns since they partnered up with Daisy. I won’t be buying any more Gamo products, which is a shame, because this old Hunter is a pretty well built rifle. The old saying “they don’t make ‘em like they used to” comes to mind. 
 
A little update. I found parts at Custom Air Seals in Australia. They shipped relatively quickly considering I live on the opposite side of the earth. Within 2 weeks I had my spring, piston seal and breech seal. I took the gun apart and found my piston had a chunk missing, and the spring was just over an inch shorter than the new one. I honed the cylinder to remove whatever snagged the seal, cleaned and polished all the parts, and installed the new parts with a light coat of black moly grease on the spring and piston. 
Wow! What a difference. It is back in business and stacking pellets at 25 yards again. I’ll never get rid of it. It was my first springer after all. It’s a little twangy, and the trigger is absolute trash, but I think it shoots almost as well as some of my much, much more expensive springers. Perfect little backyard starling wrecker.