It's really all up to you when it comes to organizing the themes of your scrapbooks. Some scrappers like to do a lot of different smaller books with themes like: Trip to Disneyland 2005, Jesse's year in baseball 2003, Mark's school years, a pregnancy scrapbook, First Year scrapbook, etc.
Others will scrap a chronoligical book for each child. For this kind of book, I would recommend an album that can be taken apart so pages can be added, as these books can get pretty thick!
Personally, I am doing a 12 x 12 album for each child, a 12 x 12 album for my dh and I, and a 12 x 12 album for the family. I also do smaller albums for our trips, our home (being built and my decorating fiascos), our doggy, etc.
I have no idea where you can get multiple albums in bulk. Maybe Costco or Sam's Club? I know that Costco used to have photo albums in bulk. Maybe since scrapping has become so popular, they're carrying scrapbooks now, too.