When you fight Schoolboy Chad, make sure to grab his number - he'll tell you when Snubbull are swarming, and that's pretty much the only time you'll be able to catch one.
In Crystal, get Lass Dana's number after fighting her, and eventually she will give you a Thunderstone.
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Route 39
There is a house with a farmer whose Miltank is sick, give it berries to make it healthier to get a reward, 3 makes it a little healthier, two more will make it a lot healthier and 2 more makes it completely healthy, (7 berries in total) then the Lady gives you TM13 (Snore)