There are a lot of considerations when looking for a host. Player count (16 slots and 24 are most common, but you have to take reserved slots into account if you want them too), ticrate, server hardware...all important in deciding who to go with.
Personally, I run a 20-slot server hosted by Wolfservers. It runs just short of $40 a month, paid primarily out of pocket and by donations from regulars. It's 100 ticrate, which is higher than most (the standard is 66) and has fairly standard server hardware (quad Xeon, 4GB of RAM on a gigabit line). I've found their support to be very good, and they actually have real humans that communicate without copy-paste responses. I'd recommend them in a heartbeat.
There are plenty of providers out there though, so shop around. The term you'll want to search is "game server provider" instead of just server host, since most of the best companies specialize in game hosting.
Hope that helps!