Any dealer will service irrespective of where you bought it, servicing is very profitable.
It's a tough market. We got a good deal by emailing 20 dealers saying:
Are you ready to sell me a Sienna XLE for $38,000 out the door pricing? If so, call me at 202-xxx-xxxx and we'll be right over. We promise we won't shop your offer around. If not, we will write again in a few days with a higher offer."
Then raised our offer $300 per email. Someone agreed to our price on our second email, and we got a really good deal.
Make sure you ask for "out the door pricing" so that the deals are comparable.
This strategy would be harder for used vehicles because you can't compare them as easily.
Some dealers won't want to play ball but if you sound like you are really willing to com