I have printed couple of posters with Posters4research aka P&D graphics.
Ordering was very easy to submit from their site and they respond very promptly. it will give you the price before submitting based upon the size and type of poster. I got it by next day morning by Fedex overnight and it was very good quality. if you live in NYC, NJ region, you can go and pick up at the same day if you want.
overall a 5* experience. Definitely recommend them for any scientific posters.