Some good places to train that would be suitable for level 30-40 Pokemon....
-- On Route 26 and 27 (the wild Pokemon are between levels 25-35)
-- On Route 47 (You will find lots of Dittos that are between level 31 and level 40)
-- By the Cliff Cave, if you go up the waterfall, there's a small patch of grass with wild Pokemon that are between levels 31-35
Hope I helped!