Instructions
Trading with a Friend
Turn on your Nintendo DS and start to play "Pokemon SoulSilver."
Get to at least Goldenrod City, after getting your first Pokemon and visiting the first few towns. Goldenrod is where the Global Trading Station (GTS) is located, and by now you will have the ability to trade between games of the same generation.
Catch or breed a Pokemon to trade away for Mankey. Consider catching or breeding a version-specific Pokemon that can be found in HeartGold but not SoulSilver, if you plan to trade with a friend using that version of the game.
Find a friend with another fourth generation Pokemon game, such as Diamond, Pearl, Platinum or HeartGold. Mankey can be captured in the wild in all of these games.
Go to the nearest Pokemon Center and head up to the second floor. Talk to the woman at the counter in the middle to trade Pokemon. Your friend should do the same.
Initiate the trade for Mankey, sending over whatever Pokemon you chose to agreed to trade away. Be sure to leave both Nintendo DS devices powered on for the duration of the trade, and save immediately afterwards.
Pal Park Transfer
Visit Fuchsia City for the first time to get access to Pal Park, which will allow you to transfer Pokemon from the Gameboy Advance games.
Turn off your Nintendo DS, and insert FireRed or LeafGreen into the Gameboy Advance cartridge slot. Leave "Pokemon SoulSilver" in the DS cartridge slot.
Catch a Mankey in either the FireRed or LeafGreen version. Mankey can be found on Routes 3, 4, 22 and 23, as well as Rock Tunnel Floor 1 and Basement 1. Make sure either to leave room for Mankey in your party or to get Mankey out from the PC after you have caught one.
Turn the DS back on and choose "Pokemon SoulSilver." On the title screen, select "Migrate from" FireRed or LeafGreen, whichever you are playing.
Enter the Pal Park in Fuchsia City, and speak with the man at the counter to begin the Capture Show. Once inside the Pal Park, go to the Mountain Area to find and catch the Mankey you chose to migrate earlier.
GTS Trading
Fight Youngster Ian on Route 34 to make sure you have seen a Mankey and have it listed in your Pokedex. If you have not seen Mankey you will not be able to use this method.
Set your Nintendo DS's Wi-Fi settings. If you do not have Wi-Fi at home, find an open access point somewhere else if possible; otherwise, you will be unable to use the Global Trade Station.
Visit the GTS in Goldenrod City, and talk to the attendant to connect to the Nintendo WFC.
Select "Seek," and search for Mankey.
Trade whatever you think is reasonable. To make your trade more appealing, consider trading away a Pokemon that is exclusive to SoulSilver, or breed a number of rarer Pokemon that are in high demand so that people will be more likely to accept your offer.