I am not sure whether you have got any good printing company for your poster printing since the post was updated during 2012.

Through online you can find number of companies that do printing services for cheaper price. But, the only thing that you need to keep in your mind is that it should have good reputation and customer feedback.

Three months back I have hired a printing company thewordprint.com.au through one of my friend reference for printing my brochure which i feel good in printing. So I also recommend you to try the same company for your printing needs. But I am not sure whether they print posters, so confirm by calling them.