There is a person on route 6 right outside of nimbasa who sells the berries, you can find the info here
For EV training you're going to want a few items:
Power items, you can get them in the PWT or Battle Subway PKRS although not needed will double EVs making it go quickly. Go to Bulbpedia and look up EV yields, that will tell you how many EV's certain pokemon give.
Getting 51 berries would be.... time consuming. You're really better off breeding and starting over, you'll save way more time, unless you do something like mass trade magnemites to get your join avenue up to buy them and keep changing your DS date for them to re-stock.