You'll be able to get legendary Pokemon as well from the past five generations. The Pawn Shop will eventually be in the Pokemon X trading board (when that's created) and after they can transfer Pokemon at the end of December they will have clones of the old legendary Pokemon you can ask for and just give them any random Pokemon for. Here's the Pawn Shop on the Pokemon Black trading board so you can understand:
The trade community is very nice. They usually will take "junk" (very common) Pokemon for trades. They helped me fill the Platinum, SoulSilver, White, and Black 2 National Pokedexes.
There is also another option which has sprung up recently. It uses a weird GTS glitch, where you enter a very specific address in the DNS settings and then you go to a special website and click on a link and it sends the Pokemon you choose to your game via Wi-Fi (Internet).
Be aware that most of the event Pokemon (Legends) will most likely be clones or maybe even hacked Pokemon (same with the GTS thing).
Probably the best bet would be get White/Black or Black 2/White 2. Catch as many as you can in the game, and then trade for the rest.
There is also a thing called the Dream World where you send a random Pokemon to bed and it has a dream. After going to the Dream World (it's an online site not in the game itself). You go to a certain area and can find Pokemon. After you play a mini-game with them, they will become your friend. Then at the end of the area, there's a Dream Tree. Put a random berry in it to send 1 of the Pokemon you met back to your game. Then wake up your Pokemon (in game) and use the bottom screen to warp to a place called Etralink then go north to find the Pokemon in a Forest. You can catch it with whatever Pokeball you want. It gives you a Dream Ball to use as well. All the Dream World Pokemon have special abilities that they don't normally have in the game.
There is also a Pokemon catching app for the 3DS. Where you shoot clouds to find Pokemon to send to your game (including the 3 "Genie" Pokemon).
If you play the TCG you can get an extra Dream World Pokemon after each season.
It should be noted that the Dream World & Pokemon Global Link (which is the website where you do this) is down until Oct. 12. To prepare for X & Y.