Sorry Dirtpilot but I'm sure that IT seats are shorter that YZ seats! However, since they are shorter you may be able to cut a seat foam down. Take your IT seat off and compare it to a YZ250 1977 seat. Maybe you can modify one. There's nothing like a new seat foam!
Seat foam is created in a blown mould (using closed cell foam) and is difficult to reproduce since it is poured into a mould to set. Open cell foam, which is commonly used in furniture upholstery, is easier to come by and use, but in my opinion not as good when exposed to the elements like bike seats. I have made seats out of open celled foam, but if I had a choice and could by a blown mould seat foam at a reasonable price I'd buy it.
Good luck