Is that what you're look for? Usually people want their personal stuff in front and might use stock image or video in the background. eg Your kid (filmed with green screen) in front of a Star Wars fight scene rather then say a green screen 'businessman on a phone' in front of your kitchen table!.
A lot depends on what type of loops you are looking for. Is it people, or effects you are looking for? If it is people, Carl has given you two really good sources but they can be pretty expensive based on the license you need. If it is effects you want like smoke, flames, spinning globes, aircraft flying through a scene, you are really talking about alpha channel effects and one source that I use for those is http://www.videoblocks.com/search.php?txtSearch=alpha&search_type=Everything Their license is royalty free and permits use for virtually anything you may need. It is subscription based so you can go monthly or they are currently offering a 1 years subscription for $99 which based on their content which includes effects, stock video clips, sound effects, and production music is pretty inexpensive. I think they also still offer a free 7 day trial.
The thing I like most about them is that their license is simple to understand and very flexible in terms of use.