Unlimited mileage is easily available from most car rental companies in the United States when you rent and return at the same location. You might have to try different rate codes.
I rent from National Car Rental. According to National's rules, rentals "originating in the United States (US) can be driven throughout the United States (US) and Canada." I have never had a geographic restriction imposed by National.
Are you looking for a one-way rental?
When you do one-way rentals, you can sometimes get unlimited milage if you stay within a geographic area, such as the Chicago metropolitan area or usually even the whole state of Florida. (
Enterprise local offices tends to be an exception because they want cars returned to the renting location.)
When you want to take your rental car long distances one way, one of two things tends to happen, depending on the company and rate code: Either the rate jumps way up (such as $1200 per week with unlighted mileage instead of $300 per week) or the weekly rate remains the same, but there's also a per-mile charge (such as 27 cents per mile or 35 cents per mile) in addition to the base rate.