There are some HDR galleries here, which are under the Creative Commons license, so you could use these for experimentation. They are already in .hdr file format.
There is a series here of raw JPGs you can use (see the related files below this one). Also Creative Commons.
Also if you download the trial versions of some HDR programs, they will come with sample images you could use. Oloneo for example.
Finally, if you look for The HDR Book by RC Concepcion, he publishes sample files on his website. They are intended for buyers of his book, but he provides a license to use them in practicing HDR techniques - if you are a student using them for educational purposes it would probably be fair use to use them. He's a nice guy, you could ask permission :)