Its almost impossible to find free ones, but if you are willing to pay, there are plenty of resources there, just google it. The best thing to do is probably take a day to photograph people at a park or somewhere related to your rendering and just cut them out from your photographs, its free.

There are lots of good, free images, especially if you are willing to put a small amount of effort into improving them as well. It is true, however, that entire libraries are hard to find for free. I would suggest working on building up your own.
Take your own photos. Search for specific items (i.e. don't search "stock people", "vector images", etc.). Also, I'm assuming you're in school, so ask older students if they have good sources. There very well may be a collection of images that has been passed down through the years. Also, make a class library; this way you have the power of all your peers' searching in addition to your own.

I like Stock Exchange and have used many of their people/tree/car images to lay the foundation of my own library. The address is:

www.sxc.hu

It is free...although the have links to images available for purchase (as it was recently purchased by another image factory which sells images).

Thank you thank you!

great resource archiTEKE. thanks.

Big or small sizes available free but daily download limit