Normally, I'd recommend that you go to Bricklink
, search for 7897 for the set you want, select the "part out" option under "on wanted lists" and create a specific wanted list for the parts from that set. You'll need to become a member if you're not already to create a wanted list and shop there. If you're not familiar with Bricklink, think of it as the Amazon Marketplace for LEGO. Lots of third party sellers hocking their wares.
One hurdle is that you don't want the whole set, you just want the parts for the car. You could do the above process, then download the instructions for 7897 and determine (by cross-referencing with the set inventory on Bricklink) which parts you could then remove from your Bricklink 7897 wanted list, but that would take some time. That seems to be the best/most cost effective method from my point of view though.
By creating a specific wishlist for the parts you need in Bricklink, you can help to minimize the number of sellers/transactions you'll be dealing with and save on shipping. Bricklink tends to be cheaper than buying from LEGO pick-a-brick online
on a per piece basis. That said, if you want 100% guaranteed new brick and LEGO has all of the parts you're looking for, you could end up getting it all in one place from LEGO. You'll have to do some independent research to figure out which method is going to be best for you. http://www.bricklink.com/catalogItem.asp?S=7897-1http://us.service.lego.com/en-US/Buildi ... fault.aspx