Any PCE (mini-DIN socket) controller will work with a TG-16 with an adapter. For regular pads, there's three options for 6-buttoners:
Arcade Pad 6 (a.k.a. - RX pad) - 100% official goods from NEC, sold with the Duo-RX and separately.
Avenue Pad 6 - from NEC Avenue instead of 'regular' NEC, but I guess it's still official (quasi official?). This is the most commonly owned pad.
Hori Fighting Commander - third party controller that's not terribly common but is well liked by those that have 'em.