Do you mean without using a Wiimote? Because that's probably still the cheapest way. PCSX2, MAME, PSX, and VR all seem to have some options for light gun support.
I would probably work my way backwards and figure out which version of the game you want to play and what emulators will support the device you want to use. If you haven't checked it out before the Emulation Wiki is great for that.
I don't know if the cheapest but I use an emulator and dolphin bar to use a wiimote as my gun to play that and any other gun game. Its really the best way to do it since you can play any game that requires a gun on any tv.