well I would assume iStockphoto.com if you have to buy them. You might want to search Christian photographers to use their images in return you'll give them credit some where on the poster or calendar? Doubt they'd go for it though.

Good luck with the bidding though. I tried my luck on eLance.com and got told I didn't have the experience to do illustrator vector images... when I've done it for over 5 years. I can't seem to get a job on there.