You can pick them up in any of the Village Centers, but I HIGHLY recommend picking one up at Race Pace Bicycles on Snowden River Parkway. GREAT shop, I've known those guys for going on ten years now. Plus if you buy something they'll usually give you the $5 map for nothing.

Columbia doesn't care about the folks that live there let alone outsiders, it's a bad town to navigate period. The trails are great for my commute because it is so short, but certainly not the most efficient way to get through town.