For national (U.S.), on site techs I can recommend http://www.servicecommunications.com we subcontract for them on the regular and they run a tight ship and have many large national accounts.  I would not get hung up on choosing someone with their own employees vs. contracting or hybrid, but more on choosing a company with experience managing projects like yours and a good project management team to run the show.

It all depends on the nature of your project, of course. There are "self service" tools like Field Nation and Work Market that you can find all your own techs and they handle 1099 and insurance, etc. for you.