Any saddle made for a Standard or Tourer will fit your Interstate with just a bit of scrunching at the nose. If the little bit of scrunching is too much for you it can be eliminated by removing some of the padding at the rise in the nose.
Any saddle for the Standard or Tourer that doesn't have the rising nose will fit. Mustang comes to mind and I, believe, Corbin.
I'm assuming that includes the stock 2000+ OEM too? May give that a try. Thanks