True, there is a legit answer here already, but here's what I'd recommend to save time.
Try a Pokemon with the Frisk ability. It lets you see the item they hold so you can save the time of trying to catch a Chansey. It's stupid how hard they are to catch. (If available, Frisk is on some Stantler and Shuppet/Banette, so that's in postgame. Odd how they'd introduce it in Gen 4 and the only Gen 4 Pokemon that gets it is in the Gen 5 Dream World.)