In my experience, most boutique development shops and agencies would love to put about 80% of their work through employees, and 20% through trusted contractors. Typically you need to be a product engineer--web, ios, etc. Expect to get paid about half of what you would for contracting work you source yourself. To get this work, go to conferences and be friendly with people in this group.
If you can't meet these people, build some open source thing they want (when I did this five years ago it was imagemagick integration for RoR). They'll contact you and pay you to customize it.
> Expect to get paid about half of what you would for contracting work you source yourself.
Being in a position that hires said contractors, I have to say this is false. Rates are competitive; we're not the one's setting them, the market is. Recruiter's commission may play a factor where it's involved, but it certainly doesn't halve the rate.
I'd also say "good luck" to acquiring contracts in your area of expertise directly from the companies that are buying them. They're looking for complete packages and the effort involved in acquiring and maintaing those relationships is quite often beyond the abilities of a single person.
If you can't meet these people, build some open source thing they want (when I did this five years ago it was imagemagick integration for RoR). They'll contact you and pay you to customize it.
I have friends who've used https://grouptalent.com/main/talent/. They had a pretty good experience.
Good luck!