I have worked through 10X Management, an agency, since 2014. Very happy with them. I'm not going to write a full review, I have left comments here in the past about my experience with 10X Management. I'm not aware of any other agencies like 10X but I'm sure they're out there.
10X Management works as a talent agency for programmers. It’s not a consultancy, outsourcing firm, development shop, or placement firm. See:
https://www.newyorker.com/magazine/2014/11/24/programmers-pr...
10X tends to represent freelancers with significant skill and experience, and/or people with rare skills. You have to already know how to work as a freelancer. They describe the process for coming on board with them (not hiring — they represent freelancers, they don’t hire them) on their web site 10xmanagement.com.
TopTal has more people and follows a more conventional outsourcing model. Their rates vary widely too. Their hiring process is somewhat grueling but several of my friends got through it, so did I. I never worked for/through TopTal though — I make more and have more control over the gigs I take with 10X.