Saying your looking for a cab file is like saying I'm looking for a exe file. It's the extension on the end. Cab files are made to be installed directly from your ppc device. You can find the greatest variety on
http://www.freewarepocketpc.net/
Personally I synce my phone to my computer. Use file explorer to browse my phone. Then just drag and drop the files you want onto your phone.
You can also just put the files onto your SD card.
Once the files are on the phone use file explorer on your phone to tap on them, and the cab will install.
Hope this helps.