I bought them from the Lippert parts store. Here's the URL.
https://store.lci1.com
I would recommend you double check clearance and ensure they will fit.
I have noticed a big difference in the amount of movement after installing the 12" pads. I also bought (from amazon) the snap pads (Xlrg) for those pads. Providing about 14" of pad area.
Since (including during heavy rains).. No more sinking into the ground.