You could do a search for clip art, or maybe check the Illustrations section of sites like iStockphoto.com, Pond5, Thinkstock, Revostock, etc.
However, I think your bigger problem will be finding everything you need at an affordable price, and not having it look like an artistic garage sale. For the cost of buying various different assets, I'd recommend you look into hiring an illustrator to do some drawings for you. Just like After Effects animators, there are droves of Illustrators and cartoonists out there that could achieve a style similar to that for a very reasonable price (certainly comparable to buying stock elements).
I'm afraid I'm not a good resource to direct you to that talent, but I'm sure someone here may have a favorite illustrator (or site). Many illustrators are familiar with drawing for animation, and keeping elements split up, ready for animation. I think you'd be a lot happier with the results.