Hate to break it to you, but the vast majority of events were limited-time-only, so you won't be able to get them yourself even if you buy the old games. There are a grand total of 15 event-only Pokémon (Mew, Celebi, Jirachi, Deoxys, Phione, Manaphy, Darkrai, Shaymin, Arceus, Victini, Zorua, Zoroark, Keldeo, Meloetta, and Genesect). Of these 15:
- 3 are curently unreleased (Keldeo, Meloetta, Genesect)
- 5 were available from events that have since expired (Deoxys, Manaphy, Darkrai, Shaymin, and Victini)
- 3 require you to obtain another event Pokémon first (Phione, Zorua, Zoroark)
That leaves four remaining event Pokémon that are currently available. Arceus is the easiest to obtain, as it requires only a Black or White gameand Wi-Fi access to obtain from the Dream World. Mew is significantly more difficult, requiring either Diamond or Pearl and the WiiWare game My Pokémon Ranch. Jirachi is even harder to obtain, requiring an English Colosseum Bonus Disc (but not Colosseum itself) and an English Ruby or Sapphire game. Celebi is by far the hardest event Pokémon to obtain, as it requires a Japanese Colosseum game, the Japanese Colosseum Bonus Disc (which unlike the English disc has limited uses, so good luck finding one that still works), and a Japanese GBA game.
Simply put, you can't get them all yourself, only those four (barring any future events), and of those only Arceus and possibly Mew are even remotely feasible. You'll have to trade for the rest.