Normally you can only get one of these by purifying and killing Baal at The God's Ordeal. If you return to the map afterwards, Baal no longer has the Robot Suit equipped.
The exception to this is if you have a full inventory when you purify and kill him. Instead of dropping the Robot Suit, Baal joins your party with it equipped. Now when you return to the map, the Baal there still has a Robot Suit. You can then repeat the process to get as many Robot Suits as you like.
The glitch also works on the Zombie King in the Cave of Trials, should you wish to get multiple Godhands. However, you will not be able to access The God's Ordeal until you get the Godhand in the normal way.
Note that this glitch was partially fixed in the PAL version, preventing you from getting more than one Robot Suit. You can still use it to get multiple Godhands though.