In Nimbasa City, the small and big arenas have trainers with Chansey, a Pokemon with a very high exp yield. The trainers you battle will often give you items.
Another method is by battling Audinos. They are found at the highest level in routes 1 and 18.
And of course, you could just keep grinding through the E4.
(And of course, it's faster when you use a Lucky Egg, but you knew that :D)