At this point in time, you have three choices.
You can use one of the existing PC guns - the only worthy one being the ACT-LABs gun (see reviews of PC-lightguns
HERE). These are pretty expensive ($100 range), and at the current time you either have to get them on ebay or sign up for a pruduction run that may or may not occur.
Your second option is to wait for the camera guns to come out, they are supposed to sometime in december. The good of these are they don't require a specific screentype. The bad is they are very inaccurate and cost $79.99. And just like the ones above you have to wait.
The best option is to buy a Guncon2 compatible gun (yes, the playstation 2 guns) for which you can use a driver on the PC. With the amount of PS2 lightguns around you can choose from any style you want. See
HERE for more information. There are a couple of reasons not to take this approach: the drivers are not commercial so there is no commercial support, and the guns do not work on a PC monitor (with 31khz horr refresh) without a hardware circuit (although this circuit is now available). They do work on arcade monitors, CRT TV's, and even some of the PS2 guns work with projection TV's.
Do your research carefully, and feel free to ask any questions you may have!