I believe it was designed to work with the Icom AT-100, AT-150, AT-500 or AH-3. It's been a while since I had an IC-735 so I'm working from memory here. It is in the manual, and the manual is freely downloadable from Icom here: http://www.icom.co.jp/world/support/download/menu.html
All of the above tuners are pretty readily available on the used market. You'd only want the AT-500 if you were planning on adding an Icom IC-2KL linear at some point in the future.