Hi, this thread will detail what Pokémon are event Pokémon and where to get them. Obviously, some !SPOILERS! may be included in this.

Please note, this is correct and as complete as I am aware of. However I may have made mistakes or omissions by accident.
   Firstly the starters:
Bulbasaur: Lilicove Pokémon center, go up the stairs, to the right, and go down the ledges to find bulbasaur.
Squirtle: (acro bike, Surf, waterfall required) Weather institute, go across the log bridge, go down and surf. Go up the waterfall and cross the acro bike bridges, cross one of the ledges and Squirtle, as well as some berries, is there.
Charmander : Route 120, near the bug maniac. (You should see him on the play through of the game anyways.)
Chikorita: Enter Petalburg woods and go right, then when you see an exit, go down through the exit, go further south and you shall find chikorita
Totodile: Dewford town, just below where Mr Briney drops you off, then go East until you see Totodile.
Cyndaquil: (Rock smash may be needed) Verdanturf town, go into the tunnel between Verdanturf and Rustboro. Go north, then west along the tunnel. If the rocks are still there, break them. Go further west and you should see Cyndaquil
Treecko: (Rock Smash required) Exit Meteor Falls (South Exit) and go south until you see a rock smash-able rock. Break this, and go East then south. Treecko awaits!
Mudkip: Inside of Meteor falls (South Exit/Entrance), go inside and mudkip awaits to the east.
Torchic: (Mach Bike required) The Old lady's rest stop, go up the mach bike slope, then east and south. You can see Torchic from the rest stop if you go to the South East corner next to the rest stop.

Now For the Fossil Pokémon
Kabuto and Omanyte: Mossdeep city, go in the trees to the south west of the city you will see Kabuto and Omanyte.
Aerodactyl: In front of the Pokémon league.
Lileep and Anorith: Get the fossils in the mirage tower, and take them to Devon Co. one of the scientists will turn them into the Pokémon Anorith and Lileep.

Legendaries
Articuno: (requires waterfall) Meteor Falls go up the waterfall, and you should see Articuno.
Zapdos:Victory road, near the near the trainer guarding the full heal.next to some water and between two waterfalls.
moltres : Pacifidlog Town. Go north to find Moltres
Mewtwo: Battle frontier, go through the smeargle cave, and when you find the carbos, go East and north to find mewtwo.
Mew, Ho-oh, Lugia, Latias/os, Deoxys: Upon entering the battle frontier you will see four items on the ground, take these to the ferry in Lilicove to go to the islands for these five legendaries, Ho-oh and lugia are on the same island.
Jirachi: Mossdeep city, also another one is located on Birth Island (Deoxys)
Latias/os: After completing the elite four, you will go home and see a thing on the TV, you choose red or blue to decide which of the latias/os pair is the roaming legendary. The other can be obtained as detailed above.
Groudon: After the E4, go to the weather institute they will tell you there is a drought somewhere, go find this place and the new cave that has formed in it. Groudon is inside this place.
Kyogre: After the E4, go to the weather institute they will tell you there is heavy rainfall somewhere, go find this place and the diving pool that has formed in it. Kyogre is inside this place.
Rayquaza: You can face rayquaza as soon as you find wake him up. Not sure if this was a glitch or purposeful thing done by the creator of the hack.
Raikou: Sootopolis city
Entei: South of Mt. Pyre, there is a route with some ledges keep as north as you can while travelling west, but you do not need to cut the trees down.
Suicune: Shoal Cave.
Celebi: Next to Scott's place in the Battle frontier.
Other Pokemon:
Eevee: Slateport city, the north of Slateport.
Lapras: East of Lilicove

Good luck finding and catching all of these

Or with Cyndaquil you could just enter from the Rustboro side and not need to use Rock Smash