There are several agencies specialising in voice artists, which can
provide you with suitable aritsts, including meeting specifications as
to gender, accent, etc. Most artists can send a digital file through
the Internet. That means, that even if you're located in the US and
would like to have an announcer with a British accent (or who usually
resides in another area in the US, this is possible).
Here are several such leading agencies:
<http://voice123.com/s/voice_over_artists.html> is a voice-over
marketplace, where you can post a job for voice artists and get them
to answer you.
<http://www.commercialvoices.com/> - a voice-talent agency
Rabbit Vocal Management
You can also contact artists without the use of an agent:
Several private British professionals are listed here:
DC Douglas - My Voice Over Guy
Only for your ineterst, I add this article about the current trend in
using voice over talents in commercials, although I think you're not
going to hire Julia Roberts:
Seth Stevenson, "The Voice-Over Gets a Makeover", Slate, 28 March
I hope this answers your question. Please contact me if you need any
clarification on this answer before you rate it.