I have used soap solutions many times at the restaurant I used to work at. Doesn't work as good in cars though because so much is not so easy to reach. I even have a bottle of special A/C Guy soap called "Big Blue". I was in an A/C club a couple years ago but because I don't do this stuff with any regularity I'm just not in it any more. Those guys really don't like talking to ametuers.
They also insist that you put r134a or any gas in in liquid form instead of right side up gaseous form.
I'll think on it a couple days. I think the dye system and maybe a cheapo modern leak detector. There are some around $50 and better ones are only $120.