Natural Selection is pretty complex (in bot-terms) so it's hard to find bots for it, and even harder to find good bots for it.
I use RC-Bot.
Personally, though, the only maps bots are good on at NS are maps where the Marines and Aliens are practically in the same room together. The bots don't hunt the map well.
The only use I really found for bots is doing testing, and learning. They're great for learning how to use the CC, 'cause you can't really use the CC without other players, and no one likes a n00b commander online so you kinda HAVE to play alone to learn.
With RC-Bot, definately alter the ladder-refresh setting to something like 20ms, instead of the default 300ms, else the bots will get stuck on ladders constantly (they always get stuck on ladders and lifts, but that's not the point).
You should also vamp up their skill levels really high, and adjust their upgrade-ratios so they do more than play Skulk and Fade all the time. Also, find really good wapoints, or make them yourself. Only play VERY simple maps with VERY straightforward waypoints.
You can also use an admin command to switch teams while still playing. I found this great for learning, as I could join one team, build them up to a massive fortress of control over 80% of the map (since the bots are usually so confused with the map they can't play) then I would switch teams and practice attacking all that I had setup. This is great for practicing scenarios, like "what to do with a wall of alien OC's," or "practicing eating marines as Onos," or "what's the best way to secure a resource point."
Other than that...bots in NS suck. It's just a bit too complex.
