Your choices are pretty limited because of the wide backbone on the DOHC frames. Other DOHC tanks from 79-82 will fit. I think the F tank looks pretty decent if you cut the tabs off.
If you're looking to buy aftermarket, there are only two options that I know of, and neither are cheap. You could pay too much for a composite tank from Benji's that is of questionable quality, or you could kick in a couple hundred more for an alloy tank from Roc City Cafe. He's got a buck for the DOHC frame backbone. I think the custom may dip a bit more in the seat area than the K/F models though, but it should still fit.
http://www.roccitycafe.com/
There's also an ebay seller (neuxstone) that supposedly has some tanks that fit, but I haven't heard of anybody using them and I don't want to be a guinea pig.