I would have to say you could battle the Reporters, Abby and Ty, over and over again. They go to three different spots. Route 118, route 120, and route 111. They go in a triangle. It's a double battle so if you want to train two pokemon at the same time, it might be ideal. Their pokemon only go to level 39 max though, but if you don't want to battle the elite 4 (I spend tons of money on items, I always have to heal after battling the elite 4, well almost always).
Abby has an Exploud and Ty has a Magneton. The Magneton is a bit tough, so I knock that one out first, the Exploud isn't that bad, it's easy as long as you don't try to use ghost moves on it.
If you really don't want to do that, then I would just suggest the Elite 4, and use the money you get from beating them to stock up on items, if you have some trouble beating them. Exp. Share works wonders here.