If you want Fisher T-tops (which are by far the best T-tops) then the easiest and best conversion is to have a complete Fisher T-top roof swapped onto your car by a body shop....you can't really "fit" Fisher tops into a standard car roof.
If you want aftermarket T-tops, they can be fitted into the standard car roof.....just they aren't as good as Fisher tops (IMHO) and the parts can be a lot harder to source (read $).....there are Hurst Hatches, American and C&C tops.....those are the ones you'll most likely find for sale or on parts cars.