I use a Lowerpro shoulder bag for my Nikon D3100. It's nice and padded and the size of the two inner pockets are infinitely adjustable, meaning that I can adjust them to any size and ensure my spare lens is very secure. It's also got plenty of outer pockets as well. The inside is a very bright orange so I can immediately see anything out of place (like lens caps!) that I might be looking for, even in low-light situations.