Do you want to build a dedicated server and run it off your own highspeed internet connection, or do you want to rent one?
I can give you advice on building the server if that's what you want to do, but I'd recommend that you had a high bandwidth internet connection, with as much upload bandwidth as possible. If you're in the UK from as little as £13.50 per month
https://www.bethere.co.uk/ offer some of the best bandwidth around, up to 48mb download, 5mb upload (under certain conditions, usually up to 24mb down and 2.5mb up which is more than adequate for hosting a game server)
The high upload is essential for a game server... By comparrison, virgin with their fibre optic lines with 20mb down only have 512kb up (Someone might want to confirm/deny this, I haven't been with virgin in quite some time...)
If you wanrt to rent a server for a monthly fee you probably want to get in touch with one of the hosting companies such as (I have no way of knowing if the following companies are any good, they're just a couple of results I got from google):
UK:
INX-Gaming game servers host. Clan servers and UK dedicated gaming server provider - Counterstrike Source, Call of Duty 4, Team Fortress 2 hosting - INX-Gaming or
Multiplay - Welcome to Multiplay, Quality UK and European Game, Clan and Dedicated Servers
Stateside:
FPSGameServers.com - Game Servers - Game Server Hosting - Game Server Rentals
You could even get me to build/possibly host the server, but that depends on location and finances...