Here is how you can check for one-way rentals: If you are travelling cross-country, choose a mid-tier or franchise agency in your Pick-up city or airport (an agency like USave, Ace, Payless or Advantage). After selecting a vehicle, you will see an email Inquiry button lower on the rental page with Vehicle detail & options, where you can contact the Agency directly to ask about one-way rentals to other cities that they serve.
If you are staying in the region or in the same state, many agencies may offer one-way rentals. Send them an Inquiry, as above.
Please note that Car Rental Express rentals generally cannot be one-way rentals. CRX affiliates are independent car rental agencies. Returning the rental car to other than the renting city is in most instances not possible.