Temporary insurance in USA - anyone had experience?

Hi all,

Me and a friend are going to the US in May this year and are planning on buying a car while there (pre 69 year, V8, set budget etc) then driving from Chicago to LA on Route 66.

Question has come up of insurance, we will need to insure the car temporarily but have no idea of how to do this? Do we just use a broker when there or is there something we can set up from this side?

Thanks in advance