Poffin Cases were introduced in Pokémon Diamond and Pearl, and they are not available in previous generations, including Soul Silver.
In Soul Silver, you can still make Poffins, but you will need to use a special item called a Poffin Mixer.
Here's how you can get a Poffin Mixer in Pokémon Soul Silver:
1. Talk to the Contest Girl in Goldenrod City: She is located near the Goldenrod Department Store.
2. Complete the Contest Spectacular: This is a series of four contests in the Goldenrod City Contest Hall. Winning all four will reward you with the Poffin Mixer.
Once you have the Poffin Mixer, you can use it to make Poffins by visiting the Contest Hall in Goldenrod City.