Not everyone has a shop around the corner that can provide poster printing services. Often, very large posters require special printers that are very expensive. Smaller towns may not have a local print shop that sees it as worth the investment. If you need a poster printing service and can't find one locally, look to the Web for your solution.
A special order printing service such as poster printing is often far less expensive from online operators. They can typically accept your design in a digital image, send you a proof, and then finish the job in a fast, inexpensive fashion. Get your design taken care of and then hit the search engines. Find the right price for the right material that you are looking for. There is a printing service for your needs and they do want your business.