I looked for a long time for a patch panel for the area under the seat. No luck and that was a year ago. You have two options, get donor metal or create your own.
I called around to a couple junkyards for donor metal, but they wanted to sell me the entire left and right side including the hump up into the trunk. Asking price was about $300 and then add truck freight.
If you have access to a bead roller, making your own patch panels is the way to go. That area isn't too complex to fabricate.