Car rental companies websites:

https://www.avis.com
http://www.nationalcar.com
http://www.hertz.com
http://www.thrifty.com
http://www.budget.com
http://www.dollar.com
http://www.alamo.com
http://www.paylesscarrental.com
http://www.enterprise.com

P.S. If you are under 21 or 25 and/or don't have US Driver License is it also a good idea to check Yellow Pages 

for small local companies, possible that those got less restrictions than listed here country wide big companies.