Here are the requirements for the box 3 quest in Cerulean from Gary Oak. MIght be a good idea to start collecting before you hit 50!
25 Leaf, 10 Bulb, 20 Hull, 25 Pot of lava, 10 flame orb, 35 pot of moss bug, 15 Gracidea, 15 revial herb, 28 water pendant, 18 stone orb, 50 feather, 40 fur, 10 tangela hair, 30 black sludge, 400 bottle of poison, 10 slow poke tail, 15 metal spike , 30 electrizer.
Nothing too hard or rare and you can farm multible at the same time. I.e. Leafs, bulbs, pot of moss bug, Tangela hair, Gracidea and revial herb can all be looted in some areas. Same with Sludge and poison etc.