Hello,
If the tank hasn't had the rust leak through / hole the tank, there are spray chemicals / sealers that can be applied, that will stop your problem.
You need to find a radiator-repair shop that provides this service in your area. They generally need all fittings removed, as the tank is washed in an acid to remove the rust, & provide a clean surface for the sealer to adhere to.
I don't know about the cost, but if the tank holds fuel without leaks, there's a pretty good chance of success with your original tank.
You may be able to find another fuel tank, but I'd nearly bet it will too, be one day faced with the same issue.
Always leave your tank as full as possible with fuel when you've finished a ride. Apart from water coming in from from possibly the gas station when you re-fuel, as the temperature drops over-night, any air in your fuel tank will reach a "dew-point", where the moisture in the air condenses into water, & then settles at the bottom of your tank. Regular use of Methylated Spirit / pure alcohol ( say a cap full when you get home ) will keep the water at bay, & allow it to be consumed by your motor without any rough running.
Good luck,
Rastus
