I did this for years, when living in various apartment complexes--but only for girls' diapers (male pee, other then mine, isn't appealing to me, maybe it's that animalistic "marking your territory" thing?). I wouldn't really get into the dumpsters, but if I saw a bag with pink used diapers I'd grab it and go "treasure hunting".
At one point, I lived near a beach and took many walks along it. If I saw groups / families with young girls in diapers I'd remember where they were and go back later, usually after dark, and look through the nearby garbage cans and often find them there.
The best time ever for this was when living in a small group of six apartments which didn't have a dumpster, but instead, just a common fenced-in garbage area with a few cans were we all took the garbage. Another family had a daughter who was physically and mentally disabled, probably in her late teens (but difficult to be sure because of her condition). She was in diapers 24/7, and the family--who were of fairly low income--kept her in the same ones until they were soaked, probably to save money.
So for well over a year, until they moved away, any time I took out my garbage there'd usually be a bag or two full of soaked, often loaded, "big kid" sized pink diapers with the most wonderfully intense soaked diaper smell I've ever enjoyed. I'd walk up there, usually at night, often while wearing my own soaked diapers under my shorts, grab a couple of these then go home and jerk off enough to cover a hockey rink. I was never caught doing this but, if I had been, all I had to say was "I accidentally threw away a $20 bill and was trying to find it!"
Full disclosure--these apartments were near where I worked, the garbage area was near the street, and driving by there was when I first noticed garbage bags full of pink wet diapers, and, honestly, that's the main reason I wanted to move there in the first place. It was a sad day when they moved away and I no longer could enjoy those diapers!
I continued keeping an eye out for wet girl diapers for years after, but at one point I caught a very bad upper respiratory infection. I wasn't around anyone else who had been sick. I finally realized that I'd probably caught this by grabbing wet diapers from some young girl who had this. So be careful.
These days, no real chances anymore, I'm in a neighborhood with no diaper-wearing kids (that I know of), everyone has their own garbage bins that they roll up to the curb once a week, if seen going through one that wasn't yours, somebody would probably call the cops. Alas. Still, once you're in the habit of keeping an eye out, you never completely stop looking (at least I haven't). Department stores, convenience stores, whatever, if I'm in the restroom I still typically have to go peek in the trash, just in case.