If you order them pretty soon
www.ColorForRealEstate.com can proably deliver them in time for your open house.
As an alternative you can print you own, 4 to the page, on 110 pound card stock. I do that often for small jobs.
However nothing I've found beats the price and quality of the full color, laminated post cards from
www.ExpressCopy.com. They'll print, laminate, mail with your list including postage for 40 cents each with a minimum order of 100.
They could easily do your job before your 25th open house.