How to search for a rental car along a segment of I95?

First, to answer your question: Use Google Maps or other mapping service. Scroll/zoom the map so it covers your desired rental area. Search "rental cars". It will pop up rental locations in that area, which will show where they are, so you can see if there are enough in the area to have a competitive market.

Next, look at the town name that's more or less in the middle of your search area. If you want to use rental car pricing search engines, use that city name as your location, and a search radius to suit you.

Second, I would challenge your underlying assumption. I used to live downtown and rent cars all the time. 2/3 of the time, downtown locations are the cheapest... but... about 1/3 of the time, an airport was significantly less expensive - enough to warrant the shlep out to the airport. So you can make no assumptions about location of cheapest rental. It is very situational, based on which station has what cars when.

...Because, repositioning cars inside a metro area is prohibitively expensive. You have to pay n+1 humans to move n cars.

I live in New York City. The least expensive place to rent a car in the city is at the airports. So, given your criteria, I would start by looking at Newark airport. Then, if you want to do some comparison shopping, search on your favorite mapping service for "rental car" around whatever your ideal location is. You can also try a price-comparison site like Kayak or Hotwire, giving a likely town or city, to see what they come up with.

Right now rental cars in the US are super expensive. A cheaper option might be to rent a U-haul pickup truck.

Using google maps, and search would be the best way to find a U-haul dealer or a rental car location.

As cars move around rental prices change. I would as others have said shop around.