If a vehicle is still in warranty, the dealer is your best option. Once it's out of warranty go to an after market supplier. Look around and get some recommendations from people who have been there, shop price, especially for something like a transmission. If you travel much it might be good to buy from a national repair shop that will warranty the transmission anywhere in the country. Dealer will just replace the transmission with a factory replacement and it will be expensive, but it will be rebuilt by the factory. After market supplier has the option to repair only what is damaged, hopefully saving you quite a bit of money. Also, many of the national transmission shops have found ways to make their transmissions more reliable. Defects are often found by the national outfits and they are frequently able to improve on the design.