Dealers make money off of financing in two ways: by marking up the interest rate, and by selling your loan to a bank at a higher rate than they bought it for. This is called the "buy rate" and the "sell rate."
Always try to secure financing options before you go to the dealership. This way, you can compare the rates they offer you with the rates you've already secured. If the dealer can beat your rate, great! If not, you can still use the financing you've already secured.
Good Sam is a good place to start for financing, though you may find better rates at your local credit union.