Do a search in the main forum for light guns.
Has been discussed ad nauseam.
My personal preference for CRT monitors is USB ACT Labs guns because they work like the old light guns did and are accurate to 3 pixels.
They aren't made anymore and require an s-vid or VGA connection though (separate model for each). They also won't work on an interlaced display or TVs that had fancy line doublers or post processing. The only reason I still keep my old CRT setup is for the light gun games.
The IR based guns like the aimtrak and wii controllers aren't accurate enough to use the gun sights IMO. You must rely on the on-screen crosshairs.
If you are happy with the performance of gun games on the wii, then you will be satisfied with them. I have to use the guns sights or I feel like I'm using an air mouse instead of holding a gun.
The biggest problem with the wii controllers is getting them to pair automatically well enough that it can be incorporated into a front-end.
To my knowledge nobody has cracked that nut.
Aim-Trak is as good as it gets for IR guns and as plug and play as you will find. Not sure what you mean buy "if you can find them".
If Ultimarc is out, then divemaster (a member here) at
www.arcadeemulator.net will have them (email or PM him)...or
www.arcadeguns.com has the Aim-Trak in a different housing.