I would search for calendar-quality pictures and then contact the owners of said pictures for permission to use them in said calendar; that way you would end up with a much more professional looking product.
I also completely disagree with wanting non-stock looking cars in said calendar. For me personally, a calendar full of quality photos of stock (looking) cars would be much more desirable than a bunch of pictures of modified ones. To me, a calendar of stock looking cars says enthusiast, whereas a calendar full of modified cars says fanboy. Just my $.02