Seat covers come in different materials like fabric, sheepskin, and neoprene that can help keep leather auto seats cooler. These covers can be purchased from various companies and shopping around allows one to find the covers they like best.

1. Is there a cover I can put over leather auto seats to help
keep them cooler?
There are a variety of seat covers made for cars.
Some are fabric...
http://www.car-truck-accessories.com/fabric-seat-covers.asp
...some are sheepskin...
http://www.rmsheepskin.com/
... some are even neoprene
http://www.wetokole.com/
Shop around and purchase whichever ones you like best.
(Note: above links provided as examples. There are many other fine companies making
similar products).