They probably don't know what a turtle box is--spell check is generally equally harmful as helpful.
Any shop that also supports windsurfing will have plenty of Tuttle fins. Anything will work. You don't need a deep Tuttle fin for it to work with a deep Tuttle box.
Just get a typical dolphin-shaped Tuttle windsurfing fin. Don't expect it to work well. the placement for the mast of a foilboard is generally way forward of where a center fin on a surfboard would be.