I've never paid for it (and don't have the courage to try offering money for it either) but I've blown a few uber drivers before. Be it as payment or as just a little extra, for fun. Each time happened naturally, discreetly and very late at night. Where I'm from many drivers advertise their "services" on Twitter though, so it's a bit easier to find them. I found a couple of them through Twitter, actually.
How do you find them on twitter? Would like to see what thats like in my area. TIA