Or to get your own maps, get into learning WorldCraft, or as it is known now: Valve Hammer (see the helpful links post on the Darkbasic Classic forum) or learn how to use Quark, originally a Quake map editor (also BSP) which uses WAD files for Textures, which in turn, can be found abundantly on the internet.
If you go for the Valve Hammer mapeditor then I'd suggest you pick up a copy of Half-Life Generation Box (the blue one). It includes Half-Life 1, Counterstrike 1, Opposing Force and Blue Shift as well as the Valve Hammer editor. The great thing about this is that you'll have tons of high quality textures to play with. It's in bargain bins everywhere, around 10-15 euro's (say 10 pounds) or so. Of course you can't sell any of your work if it makes it into your game, but at least it helps you get going bigtime.
Personally I find Quark easier to use, but that's probably because I've got more experience with that piece of software.
Good luck, mate
Yskonyn -
"It's better to wish down here you were up, then to wish up there you were down."
"The ONLY time you have too much fuel on board is when you are on fire."