@Taciturnu Many people think that rental companies only rent to people 25 and over, but that is actually a myth. While some still have that policy, most will rent to anyone 21 and over. It just costs more to rent before you’re 25.
When I wanted to rent a car, I used yellowpages to find out what rental companies were in my area. I then looked at each of their websites to get their rates. Since you are under 25, you will need to check their rules to see what kind of penalties they will charge you for your age. The company with the least expensive rentals in general is likely not the cheapest one for you since you are 21. Avis ended up being the best for me when I rented at 21. Given that I don’t live in your area, you should do your own research.