Hi. Not sure how much help this will be to you, but here goes.
Bought a brand new MAC tools detector off of eBay last year for about 60 bucks. Have used it several times. Basically, they have a replaceable sensor element at the tip of a flex hose. A small pump pulls air (and anything else) through the hose and therefore over the sensor. When it detects freon/halogens, it alarms. I think the MAC and the SNAP-ON are the very same tool.
My experience is that these are VERY sensitive, and it takes some skill in setting the sensitivity so its not going off all the time.
You want one that is adjustable (so you can use it for R12 and R134a) and has replacement sensors readily available. On that basis, a MAC or a SNAP-ON sensor can be had by finding the local tool guy.
They are really good at finding evaporator leaks... because you don't generally have access to the evap, all you have to do is stick the proble into an internal dash vent... if there's freon in the air stream, it will let you know.
For small leaks, I've found it alarms all over the place because the freon migrates pretty readily under the hood. For this use, I prefer an flourescent dye and a UV light. Easy to see and no mistaking the leak if lines are close together.
Good luck, KenP