To be honest I find my internships word of mouth. Although the one I have had for the past 6 summers 3 winters I got thanks to my mom. She got me started, but I got myself invited back. Ask people currently working there if they know of anyone. The websites aren't always so great. Also try university career centers. Some universities require internships prior to graduation, while others don't. What types of positions do you have available and where are you located? I may actually know some people who may be interested.
I have gotten past internships from word of mouth, and I got one off of successtrak (college version monster). I also looked at Careerbuilder and local sites daily when I needed an internship. But these may flood you with resumes. It might be easier to speak with local colleges career services or professors to get some names.