Hi M.! Installing a car seat on leather is no big deal. I saw multiple responses to check out www.seatcheck.org, and that's a great idea. Also there's a local website (seatcheck.org is the national registrar) www.patechs.com where you can find a tech in your area or attend one of the monthly (sometimes more) free seat checks. I've been a certified Child Passenger Safety tech for over 2 years now, and there are lots of other great techs in the Pittsburgh area that can help you out as well.
If you install the seat properly, and feel that the vehicle seat is too slippery, you can buy a roll of shelf liner (the grippy kind, not the adhesive obviously!) and cut a piece to the approximate size of your carseat base - the shelf liner is thin enough that it won't compromise the installation of your carseat and provide you with a bit more resistance. Do not, I repeat, DO NOT buy a seat protector. They are usually too thick and are not safe to use.
The next seat check event in on September 10th in West Deer (and I may even be there if my work schedule allows!). However, if you need to see a tech earlier than that there is a contact number on the patechs.com home page to schedule an appointment.