If you're patient enough to look in the long grass for hours, then go to the safari zone. I caught a lot, like Gligar and many others. You can also find rare Pokemon all over, like Mt Pyre for example, especially at the top in the grass, as I did find and catch a Chimecho.
you can catch sudowoodo and smeargle
catch them :P
It is very rare but you can catch it at mount Pyre.
Route 119(rare)
You can't catch a Gardevoir in Pokemon Emerald. To obtain one you can catch a rare Ralts on Route 102 and evolve in into a Krilia, and then a Gardevoir.
On Route 116. Rarity: Rare.
underwater routes 124-126,but rare.
it's just by luck
The rare Pokemon on emerald are Rayquasa, kyogre (hard to catch), groudon (hard to catch), ho-oh, lugia, sudwodoo, smeargle and metagross. Regirock, regiice, registeel, latais, latios, mew, deoxys, and serribi.
You can find nosepass in granite cave very rare though.
yu have to go to sootopolis and use dive.Beware! its very rare.
Underwater, in where the seaweeds are, but it's rare and quite hard to catch.