I can't vouch for quality yet, but I saw someone online recommend Best Value Copy. I priced printing double sided 5x7s on heavy paper (for planner inserts) and on cardstock (for dividers), and it seems pretty reasonable. There are a lot of different paper choices, so I'm guessing you could find something that would work for calendars. The shipping is a little pricey ($13-ish for my $10 print job, if I'm remembering correctly), but the total cost was still less than some local printers.