Where can I find the OTA update file?
Two pieces of ground I want to cover here:


/cache is typically a separate partition on your device, which wouldn't be mountable by a PC via USB, which is why that isn't working. The USB connection generally accesses a directory named "sdcard" (or similar) which is used as an external storage device, although this storage area doesn't have to be physically external to the device (it may just be a partition named /sdcard rather than a physical card).
If the OTA is in /cache then you should be able to find it with a file explorer or shell on your device. Once you've found it, you could simply move the file from /cache to your external storage and then copy it over USB. You'll need root in order to access /cache, though. There are a number of file explorers that support root access, but I personally use ES File Explorer.