Having been a Director out of the US who used contractors I can add some insight on this.
Contracting in the US is very different to the UK.
In the UK there is a really good benefit to operating as a contractor. You tend to find really good candidates who have a wide range of business and IT skills, as well as really strong niche skills.
Because it's more beneficial from a tax perspective, UK contractors can earn more and tend to be much better than a lot of permie equivalents.
This is not true when it comes to US contractors. In the US, companies prefer to hire top notch individuals and reward them with good salaries and especially equity. In tech companies, equity which vests a year after you start and carries on can be more than the salary.
This is because the US operates under 'employment at will', which effectively means they can let you go when they want, and vice versa you can just walk out. Mostly though you are 2 weeks out of courtesy. To stop this they like to tie the best candidates in.
US contractors tend to be a pure niche skill and tend to be those individuals who just cannot seem to get a full time job. This means they're not that good.
Having tried to hire US contractors who had my skillsets, or the skills I wanted, I received a lot of resumes of mediocre individuals. They all came via agencies. Monster was used quite a lot and one other board which I forget (remind me).
Of the resumes I received, most were at a much higher expected rate than normal. So for example a Tableau developer I had came in at $100 per hour (that was £60 an hour at the time, with the change in currency, that's closer to £80). With he change in currency I reckon the rates would be much higher.
Obviously being a US citizen or Green card holder is the only way.
It's not a big contract market, and as I said most companies don't bother because they an fire you anyway or lay off staff when they want (It's not Europe), so a smaller market.
Let us know how you get on