Harbor freight has a good electronic detector - I have one.
I bought another type - China - on Ebay $20 - that's the one I use all the time. Both are sensitive enough to sniff a tiny leak in the evaporator by sniffing the duct.
Dyes work, but it has to be a leak where you can see it - Murphy's Law says it's not where you can see it.
Be sure to check the compressor - run it for awhile, then shut off the engine and check the ends. Most of the Jeep compressors replaced under warranty are for a slow leak. They are actually easy to reseal - do a search on the internet with your compressor number - seal kits and instructions are available.
Usually TJ compressors are Mitsubishi or Sanden - very similar.
A can of stop leak in an otherwise good system won't hurt anything, no more than anti-freeze destroys your radiator - it has stop leak in it too. It often solves the problem. 2nd can is a maybe-maybe. 3rd can -- Don't!
Stop leak can plug things up if you use too much, but the alternative is replace expensive parts - by guessing. That can get out of hand.