The Legendary Pokémon
After you have gotten the Rainbow feather from the Goldenrod Radion director when the tower has been taken over by the Rockets, go back to Ecretak City and go into the northern house. Go in all the stairs and you'll eventually come to a tower. Go inside, work your way through the maze inside, and once you make it to the roof you'll find Ho-Oh.
You better save your game before fighting Ho-Oh, because like Mewtwo in RBY, there is only one. Ho-Oh is only level 40 so using the Master Ball isn't a good idea.
Yes, Ho-Oh can be obtained in the Silver Version, you just won't be able to get it as soon as you got Lugia. In Pewter City there is a man who will give you a Rainbow Feather. Take this Rainbow Feather and head to the same place as Ho-Oh is in Gold and you will fight a level 70 Houou. It is best to use the Master Ball here.
Yes, Lugia can be obtained in the Gold Version, you just won't be able to get it as soon as you got Ho-Oh. In Pewter City there is a man who will give you a Silver Feather. Take this Silver Feather and head to the same place as Lugia is in Silver and you will fight a level 70 Lugia. Use the Master Ball here.
Go to Oilvine City and surf south trying to stay as far to the right as you can. There is 4 entrances to the cave where Lugia is so if you follow the wall the first one you come to is the one you need to enter.
Once inside go to the right and choose the top path when it comes to a fork. Now you'll make your way to a waterfall, go down it, then surf up and you should find Lugia swimming in the water.
Lugia is also level 40 and will give you 1257 experience points if you beat him. But DON'T, what are you crazy or something!? Again, using the Master Ball at this time isn't wise.
If you wanted to know, Lugia learns Hydro Pump at level 44.
The Legendary Dogs
Raikou, Entei and Suicune. These three Legendary Pokémon have no specific location of where you can find them because they are always on the move throughout the tall grasses of the Gold and Silver map. Once you run into one of them you can always keep tabs on where they are by going to your Pokédex and then checking their location (the first choice at the bottom of their Pokédex entry). They will only appear in Johto and not in caves or water
But first, if you haven't already waken them up you will have to go to Ecruteak City. Go in the Tower pictured, break the rock in the way and fall down the little hole. Walk up the stairs and they will awaken and scatter.
The Unown Page
Pokémon Not in GS