[Michael.SF.]

You might try your local Kinko's, though they can be expensive.

Some office max stores offer similar capabilities, and at lower costs.

[Doctor Raoul]

Kinkos = $7.50 per sq ft on color plotter.

A 3 foot by 2 foot poster = $45.00

[boxerblues]

for large posters, check in your local phone book for printers/copier with blueprinting capabilities, they can handle the large formatted size of posters with a quick turnaround. You may have to mount the prints yourself though