Hello to you from Conifer, CO. Welcome to Air Forums and the vintage Airstream hobby. I'll bet you will find these Forums very helpful as you get started on your renovation of your 1976 25 footer. There will be lots to learn.
My 66 Trade Wind 24 footer had lots of needs when I purchased it 3 years ago. And it still has needs, but we've made progress. I recommend you do a complete analysis of all your trailer's systems so you have a feel for what it will take to get it back on the road again. I like to start from the ground up. Tires, brakes, axles, shocks, belly pan, frame, subfloor, wiring, plumbing, appliances, aluminum body, windows, et. al. Make a list of what's good and what isn't. The 70s trailers do have well known issues with frame rust especially in the rear of the trailer. Make sure you plan to drop the belly plan and inspect the frame for rust.
My trailer didn't have many working appliances, the subfloor was rotted in the rear of the trailer, the plumbing was shot, etc.
Then develop your plan to renovate. Many folks get their old Airstreams "towable" first, meaning good tires, brakes, axles, frame, subfloor and exterior lights. Then they make the trailer water tight. This creates an "aluminum tent" that is somewhat useable with an air mattress and sleeping bag.
Now you can begin the plumbing and wiring projects and restoring old cabinets or making some new ones. Eventually with a lot of time and money, your old Airstream will be like new again.
That said, water tanks are readily available. My 66 needed new tanks all around. I purchased my waste water tanks from Inca Plastics in California. They have a catalog of over 100 tanks. What I did was thumb through the on line catalog until I found a tank with the dimensions and volume I wanted. Inca supplied these tanks in good shape, no problems. I purchased my fresh water tank from Inland Airstream, also in California. The tank they sell was an exact replica of the fresh water tank that was originally in my Trade Wind (which is quite a bit different than your trailer.)
Post some photos of your old Airstream, we'd like to see it if your willing.