Um, they're in RAM... hence the name, "RAM Preview". Since RAM is volatile memory, you won't find what you seek.
Now, you CAN save RAM Previews as individual files in AE if you wish. Is that what you want?
As Dave says, and as the name indicates, RAM previews are held in RAM, not on disk.
There is a disk cache that you can turn on, purge, etc.---but that's not directly related to RAM previews.