We've used National the past three visits to WDW and found they had the best price each time. We belong to the Emerald Aisle which allows us to bypass the counter.....worth even paying a few $$'s more for the rental if need be.
I agree with the other posters about checking Mousavers.com site for codes. There is also another one: rentalcodes.com which has lots of them.
Good luck!
Sammy