http://www.blockposters.com/ You can print it out on as many 8x11 sheets as needed, then stick them on a posterboard
Walgreens does posters. Upload your picture over the internet and select which store you want to pick the poster up from. Currently they're offering 40% off. 40% Off Prints, Posters, and Enlargements Enter coupon code BIGOCTOBER at checkout thru Saturday, October 12th* They only go up to 24' x 36' but you could print it out on 2 posters and glue to poster board like Brian said.
