I'm just gonna try to be specific , so if it isn't , don't blame me. At least I tried.
Lapras:
You should have beat Cynthia already and get the National dex as speed freak said.
Go to the Pokemon league , down the waterfall , and in victory road. Cross the bridge and use rock climb to get down ( I think , because I caught a Lapras before so yeah, ) . Go left then down or press the top part of your + . You will see stairs and go up, then you follow this path since the stairs only lead you 1 way. (Or so I think ) , then you see a cave and then enter it, use defog and if this is your 1st time then Marley will accompany you with her Arcanine . Marley is the girl with a black and white striped dress. Now you have to fight some trainers while going straight ( if this is your first time ) and then go right , ( not sure ) . Again I am not sure but maybe by this time you'll see a rectangle pond with water . Surf in it , and there is a chance you will find Lapras. I'm warning you though , Lapras has ice beam , brine , safeguard and perish song. When Lapras uses perish song, you better catch it or you'll have to look for another Lapras.
Growlithe :
You should have the game boy advance game Pokemon Fire red / leaf green in the cartridge slot in your ds and you can look for a Growlithe at route 201 and route 202.
Dratini:
You should know or remember the way to mt. Coronet , and there is a waterfall on the way, Use the super rod in front of the waterfall ( don't go up it ) . And there you can find a dratini / dragonair.
Snorlax:
You should have some honey , which you can get at floaroma town and use it on any honey tree. Honey trees ( aka tress that have yellow leaves ) can be found at route 205,206,207,208,209,210,211,212,213,214,215.218,221,222,Eterna forest , floaroma meadow , fuego iron works , which is under the bridge before the valley wind works , surf to the left and go to the land and a building . There are patches of grass there before the building , you can also find honey trees at the valley wind works.Wait for a few hours , and if there is shaking , you should find a munchlax . Munchlax evolves into a snorlax by happiness. You should try leveling it up , walking a lot , battling with it, giving it berries and other things like maybe entering contests with it can increase it's happiness . When you catch munchlax, use a luxury ball and give it a soothe bell , to help with the happiness. Also you can go to the resort area, but you'll need at least ten contest ribbons to enter it . Go to the 2nd floor and you can massage the Pokemon at the top left spot. This only works once a day though. In veilstone city, from the Pokemon center , go down and then go near the gym . The house nearest to the gym is the house of the massage girl . This only works once a day aswell. Munchlax should evolve into snorlax after you keep on doing these things . Plus you can get contest items from the massage girl in veilstone.