For Pokemon between the levels 30 and 50:
Most recommend spot:
-Cycling path
-Island 1,2,3,4
For Pokemon between the levels 50 and 60:
-Victory Road
-Cerulean Cave
For Pokemon between the levels 60 and 100:
Most recommend spots:
-Pokemon League
-Pokemon rangers on island 7
-Lady Jackie on resort gorgeous ( gives 10 000 poke yen anytime you beat her)
Hope this helped :)