I'm technical, I build stuff myself, so take my opinion with a pinch of salt.

If you have built a non-technical throw-away prototype (paper, wireframe, ...), at some point you may find your testing is limited. IMO, a technical THROW-AWAY prototype is ok to be outsourced, because it'll take all the short cuts it can, and when you are done testing you want to change at least 50% of it anyway, so throwing away and starting from scratch is the right answer.

That said, I don't know if there are any agencies that do this. But you could probably find a freelancer that enjoys this type of work. You definitely need a tech co-founder when you build the real thing, though.