Very few custom maps servers exist.
There's a couple reasons why custom map servers are usually unpopulated.
To start off, you have to remember that people aren't usually willing to download maps that they don't know. trade_plaza and mario_kart are map names that you can see are not going to be very serious. However, nobody's going to look for a server running koth_quarry because they don't know what koth_quarry is. Nobody's willing to download a map they've never heard of to play probably once with potentially no-one.
And if one person doesn't do that, neither is anyone else, leaving for an unpopulated server that's up for pretty much no reason.
Having said this, there are a couple servers I know that run custom maps occasionally, one of them being the Lion's Den (games.dongues.net:27015). I know they run swiftwater every now and then, and some others, but I'm too lazy to check right now.
Hopefully tf2maps servers can get some traction and have people on them more often, because there's a lot of custom maps that are way better than official maps.