As far as I know, nobody has written a driver or coded any software to take say a Happs light gun and rig it to a MAME cab. Your best options right now are to do one of the following:
- Buy an Act Labs gun off eBay (They've been discontinued)
- Buy a light gun at Lik Sang (Not very good though)
- Wait till Act Labs brings out a new light gun (Not soon)
- Beg someone to code MAME so it accepts standard arcade lightgun inputs