It's not ideal if you need to be picky about vehicle or company, but I've used Priceline to rent cars for work a few times (I'm 23 also) and the rate I got through Priceline has always included any underage driver fees.
It also carried the advantage in my case that I could have my boss prepay for the rental with his credit card.
YMMV of course, I've not checked the T&Cs to see if this is specifically allowed, but you are required to enter the age of the driver while making the reservation so I suspect it is.