Solar controller

Hello,
 
I am looking to purchase a used solar controller. I am a high school student doing a science project building a solar cart/bike. My theme is to build it from used materials. So, I am looking for a used 36 or 48 volt controller to be used with the solar panels I have already obtained. I am currently building the bike and the frame should be completed this month. Once complete I plan to enter it in solar races and exhibit it at science fair competitions around the county. My results will also be posted on my website www.solarcarscience.com along with links to people who have gotten involved in my project. I currently attend Camden Central High school and will be entering the 11th grade this fall.
 
Thanks in advance for any advice or assistance you will be willing to provide,
 
Jesse Clifft
2495 Old Hwy 70 East
Camden Tn, 38320