Weve had mixed luck with Costco's posters. Inexpensive but sometimes color is off
FedEx/Kinko's actually did some very good quality 20 x 30 posters for me recently. Not cheap, but I only needed 2 for a customer.
I use PSPRint for my greeting cards. They do all the other things as well.
Ron Baker sent me this link: http://www.uprinting.com/He is trying them out.
I get stuff done at Allegra print and imaging
I've used Docustar with good results.