I think you have either to choose freelancers or Aircom. There might be no others, Aircom they have offices all over the world, the have the best planning tool Asset3G but they are not good in RF consultancy because what they do is called contracting services. In their website there is a page for contracting, you can upload your CV and they will hire you if they get a project somewhere which fits you. This results a lot of complain from the customers because sometime they will send you someone who have no great experience in the field.