You said piston so many times my head is spinning, the proper term is cylinder
Sounds like you have a 1"-1.25" diameter by 24" stroke cylinder such an animal will not be cheep to purchase (prob at least $70.00 or so.) You could try and find a local company that could get you one at costs. Search for pneumatic supplies.
I have a 1.5" and 2" in the shop and we will more than likely be using the 1.5" cylinder for our stinger. Can you fit a 2" cylinder and provide it enough air?
What was leaking about the first cylinder? If it was a fitting, those can be replaced. Some cylinders can be rebuild. Can you post a picture of find a part number?