Instructions
Go to the Kecleon Shop in town or any of the Kecleon Shops that appear in the dungeons. The Kecleon Shop inventory changes every day, so check back every day to see whether it has a Sun Ribbon. The inventory also varies from shop to shop, so you may not find a Sun Ribbon at a shop in one dungeon, but it might be in another.
Use the Wonder Mail system at the main menu of the game to enter a code. These codes give you new missions with special rewards. When you go into Wonder Mail, you can use a virtual keyboard to enter codes. When you start the game, the mission will be available.
Enter "2XF=K#9N&XKRY9F+R58Y-88MRR6#&OFF#T" in the Wonder Mail system to get a mission with Magikarp. You must explore the B15F area of the Concealed Ruins with him -- your reward is a Sun Ribbon.
Enter "40TP-4Y-&38WFXXX6N3RKY7SJ7X2%&TK15" in the Wonder Mail system to get a mission with Electibuzz. This mission takes place in the B17F of the Surrounded Sea dungeon and you must go prospecting with Electibuzz. The reward is a Sun Ribbon.
Enter "F#@135QC2CF9278QRXQN+4WQ8PSY2WWHPT" in the Wonder Mail system to unlock a mission wherein you must rescue an Oddish. This mission takes place in the B6F area of the Crystal Cave, and your reward is a Sun Ribbon and 350 Poke.