I got my inverter (when i was using one a couple of years ago) from a local car shop to me (at the time), called GK Motor Factors in Colchester.
However, I have since moved on from the inefficient inverter route, and have gone to DC-DC.
Ultim8pc have the Morex DC-DC PSUs, which is what I am currently using, however I am going to be getting myself an Opus soon. I don't trust it putting un-regulated power into my desktop hard drive (although it has been OK for a few months, i don't want to risk it longer than i have to!) and i'm starting to miss having a proper shut-down controller also!
I will possibly be needing to get 4 (2 for me, 1 for boss, 1 for dad) within the next couple of months, so I might organise a group-buy for UK people