Again, the answer is yes. For every single Pokemon Game, you can actually use a cheating device and thus enter cheat codes to obtain all the known the Pokemon. These devices are known as Gamesharks and Action Replays, and they can be bought usually online for not an outrageous price. Gameshark Codes can be used on the first and second generation Games, while Action Replay Codes can be used on the third, fourth, fifth, and sixth generation games.
The process of entering Gameshark Codes and Action Replay Codes is actually a relatively simple and easy process. If you have a physical copy of your early generation Games and the early handheld systems they were played on, you can easily hook up the Gameshark Device and begin entering codes. Most Gameshark, and even Action Replay, Codes first require a Master Code, so keep that in mind before entering other codes. Also make sure the codes are actual working codes, as a bad code can serious damage the saved file on your games. If you are playing an online version of a game, on either a Visual Boy Adavance or My Boy Emulator, there should be a place or tab to access entering codes and the process is the exact same when physically entering Gameshark Codes on a copy of the game.
Action Replays work in almost the exact same way as the Gameshark does, except it's for the newer Pokemon Games. Most codes also require first entering a Master Code, and bad, non-working codes can disrupt saved game files. Everything else about the process is basically a carbon copy of the the way Gameshark is used, on phsycial game copies and virtual ones too.
Need some codes? We have one of the most diverse selection of Pokemon Codes on the internet!
Pokemon Red and Blue Gameshark Codes
Pokemon Yellow Gameshark Codes
Pokemon Gold and Silver Gameshark Codes
Pokemon Crystal Gameshark Codes
Pokemon Ruby and Sapphire Action Replay Codes
Pokemon Emerald Action Replay Codes
Pokemon Fire Red and Leaf Green Action Replay Codes
Pokemon Diamond and Pearl Action Replay Codes
Pokemon Platinum Action Replay Codes
Pokemon Heart Gold and Soul Silver Action Replay Codes
Pokemon Black and White Action Replay Codes
Pokemon Black 2 and White 2 Action Replay Codes
But do try to remember that the newer Pokemon Games are becoming advanced enough to catch non-legit (fake) Pokemon in their Wi-Fi Filters, so you might not be able to trade or transfer Pokemon that were obtained through code if they have even the smallest, inconsistent niche.