Most girls who are UTR are usually working in some other form of sex work or have worked as an escort before. It usually takes more work to find since they are not openly advertising or are RARELY advertising. Here are a few ideas.
Strippers/Strip Clubs: This is usually the easiest, especially if it is a club known for "extras." Chris Rock had that song "No Sex in the Champagne Room" but the truth is plenty of stuff goes down in the VIP/Champagne room. Even if its a club that is strict you can find girls willing to make "arrangements." Just go to a strip club and do strip club things. Talk to the girls, flirt with the girls, build a rapport and get dances. If its a strict club, be a little slick and coy. Flirt with her for a while and take your time. When you're getting a private dance and nobody else is around ask her if she makes arrangements. Be discreet and recognize you are potentially fucking with her reputation and job. She can get fired if management finds out she is fucking clients. If she is willing and trusts you she will say yes. If she isn't into it, take no for an answer and move on. If the club is known for extras the girls will usually proposition you openly. You can just get a dance and ask them if they would meet outside of the club. You will usually save money because the club takes a huge cut on VIP/private dances.
Brothels/Massage Parlors: If you have a good reputation with a girl at a brothel or massage parlor you can try to ask her if she would meet outside. Again, if she gets caught doing this it could get her fired but sometimes girls will transition to UTR this way.
Regular Dating Sites/Sugar Dating sites: I'm not sure if sugar babies count as UTR, but you can find some girls moonlighting there. Also sometimes you will find girls on regular dating sites like tinder or match. If you're into TS girls, you can find some on Grindr and other gay hookup apps.
Social Media/Instagram: This is the most risky and takes the most work. If you see a cam girl or only fans model you can shoot them a DM on their Twitter and ask. However this could result in blowback because girls who don't escort hate being asked about it. UTR girls will usually have social media and follow a bunch of sex workers, so if you're horny on twitter you can just check out some escorts "following/followed by" to see and you might find a few UTR. I've heard some guys reach out to Instagram models/THOTS as well. Social media models always talk about being "flown out" by rappers or men of means, they are obviously getting paid to fuck but I'd expect them to be expensive.
Advertisements: Yes, I know UTR means that a girl usually doesn't openly advertise but from time to time they post an ad. It is usually on the crappier sites. You just have to cruise ads and keep your eye out and see what pops up from time to time.
I wouldn't recommend approaching random civilians and propositioning them. That is pretty creepy and you'll probably run into trouble. However I have heard some stories of guys getting a UTR girl just like this. Either some random bartender or waitress they proposition and end up having an ongoing thing/affair. I've never done this and probably never will. However maybe you're at a bar and the girl serving your drink is a hot, young single mother who needs a few extra bucks to pay some bills. You can ask but you'll probably get asked to leave. You have to be real slick and coy to pull something like this off.