So i just got my 92 SC400 and got a problem with the fuel tank because the car was sitting in the sun for 15 years so i been trying to find a new one or at least a decently clean one but i cant seem to find one so if you have any suggestions on what should I do or where to look it up I will be grateful

Have you isolated the problem with fuel tank? I would not think a fuel tank would not go bad sitting in the sun for an extended time. What about the filler neck going to the tank?

Took it to a shop to check on it and the guy told me you can’t use this tank showed me how was the fuel tank from the inside it was looking all muddy and rusted

He is just trying to sell you a fuel tank. I have cleaned several old BMW tanks with a couple different methods. Cleaned one with solvent and a chain. I shook the tank with the solvent and chain until I had it clean. I must warn you it takes some energy and you might want to have friends over to take turns in shaking. After cleaning and emptying out the contents wash out with a good stream of water. Then shake the water out and then pour in rubbing alcohol and then shake real good, must coat surfaces to insure no rust. Leave it overnight and it is ready to go back in the car. Another method is to substitute solvent for muriatic acid. This is a product that used for cleaning concrete swimming pools in the spring. When i used this method I wore a mask and rubber gloves. Don't know how sensitive you skin is. Clean the same way as above. If you have a friend that has bore scope you could look at the inside of the tank after cleaning.
This should save you some money so you can spend it on something else for you car. It would be a good time to change the fuel filter. It should be under the car on driver's side.
Of course you just use a couple of quarts of solvent or muramic acid.
Good luck

Get a second opinion on it. I used this:
https://por15.com/products/fuel-tank...yABEgLzUvD_BwE
To clean and reseal the original fuel tank on my ‘65 FJ40 9 years ago and it’s still great.

This is a great suggestion also to line the inside of the tank with a sealer especially if you were to have some pinhole leaks that you are not aware of. Por 15 is supposed to be an improvement over what they used on fighter airplanes to seal bullet holes during combat and of course you would not have that problem. Which ever path you take don't forget the new filter.