Well, for starters, look online. Most leagues have websites-- why not contact the moderators at a few and simply ask for help?
Start here (Sorry, I led with what I know-- NYC area leagues. But you can certainly google ones close to you.)
https://www.facebook.com/National-Catholic-Forensic-League-316130124338/
http://www.ncfl.org/ncfl-resources
http://www.lifa.org/
http://www.msdlonline.org/related-organizations.html
http://debate.nyc/
http://www.phillycfl.org/
http://www.pbcfl.net/
http://www.oocities.org/geneseevalley/contact.html