A good way to come to the US is to join a mission sponsored by one of the country’s trade organizations or local incubators. You will come over, be introduced to a number of resources locally and decide who might work for you. These resources will range from attorneys, accountants, marketing experts, and VCs. There are a number of law firms that specialize in immigration - www.mj-law.com is one of them.
It’s unrealistic to expect that you will find a “marketing” resource who will want to invest cash or sweat equity in your company so it’s critical that you have funds to bankroll your expansion to the US. We’ve benchmarked (http://www.slideshare.net/visionandexecution/entering-the-us-survey-resultsv4) this and the companies who are most successful get funds for expansion in their home country and then raise more funds after they’re here and have US sales traction. In other words there’s a poor likelihood a US investor would invest until you have US sales.
For market expertise they are tons of consulting firms/consultants who focus on exactly what you need: local market expertise to gain traction in the US. In our benchmarking study we found that companies who hired local resources (paid their consulting fees) fared the best. My firm is one of those consulting firms specializing in helping companies come to the US.