Usually you can instruct your (color) printer to print in black and white, and it will print in various shades of grey.

You can also do this in acrobat (print to Adobe PDF using black and white)

Typically, people print large posters at places like FedEx Kinko. Converting color posters to B&W ones, although sound trivial conceptually, sometimes poses a practical challenge. One is you don't exactly know how the color posters will look in black and white.