Road trip, insurance and which to choose

Myself and 3 of my friends(all of us 24) are planning a road trip in the US. I’m not a US citizen so traveling down for the first time- the other 3 are.

We’re doing a road trip from SD to Seattle and then back down to SFO. Due to some age related issue, people above 25 get a better deal.

We don’t own a car in the US- so what is the best way to get an insurance, which insurance works well for this?

Any help is much appreciated