You can buy them at Babies R us.
If you look on the side of your carseat, there should be info in where you can order the model you need
You can get them almost anywhere, purchase the carseat &extra base at the same time. Ours is from Buy Buy Baby and was suggested we do that by the sales person because we didn't even think or the fact we have multiple cars. However we picked up a 3rd at a local baby sale hosted by a group called Mothers of Multiples that we found on Craigs List under Garage Sales.
Walmart and the actual site for the brand of seat u have