As I love comics, my theme on the calendar was easy. I used some pictures from newspapers I got in a comic book store in New York. And by using those pictures I got a lot of colors, so I kept the base color neutral. So nature colored paper and some matching organic yarn just to add an extra detail.
Try to use material you have at home. Some old postcards or newspapers, gift wrapping paper or a photo you love. There is also a lot of internet sites that allows you to print their art out for free.
And if you haven't got some company giving it to you for free check with people you know, maybe someone got one to spare. If that doesn't work you can buy a really cheap one and just give it a makeover, so you'll save money either way!