A nice little article in which Jeff Sauro lists his 5 reasons why he likes remote testing. He lists a few services he's involved with too, so if you're new to this kind of thing there are a few companies to investigate.
I like the article and agree with 4 of the 5 points. But beware on point 4!
Jeff says "You have the ability to collect data without the need for a facilitator's time."
True enough, but you still need to be able to write the tasks in the first place. When talking to colleagues, I've advised them to try user testing for themselves first before making use of remote services.
If you screw up when facilitating yourself, you always have a second chance. If you find you've led the participant with your question, you can halt an activity and set a new task. Or if your question hasn't uncovered what you'd hoped for, you can ask follow up questions. When you use remote testing, there's no second chance - you set your test task(s) and then get back your videos/data/feedback with no further involvement.
Of course you can disregard my advice if you can afford to make mistakes and spend more retesting.
5 benefits of remote usability testing - article by Jeff Sauro