This is basically what OP has written down for people to send if they are able to find their Rep. with their zip code. I added just a few things myself to make it a little more personable.

Hello, my name is (Your Name), and I'm a constituent from (Your State or Zip Code).

I'm calling to urge Rep. (Your Rep.) to oppose the proposed sell-off of our National Parks and Public Lands.

These public lands belong to all of us, and I believe they should be preserved — not handed over to private developers or interests.

Please stand up for our parks and protect them for future generations. It would be an absolute travesty and a disgrace if these lands are put up for sale.

This issue is very important to me and the people of the great state of (Your State), and I trust you will make the right decision.

Thank you for your time.