Want to add an on/off switch via SPDT 30A relay and am having trouble finding female blade connectors around my local electronics shops or online.

TIA!

What kind of 8 AWG wire are you using? Can you solder the wire to the terminal and heat shrink over it? Can you squeeze it into a non-insulated 12/10 female connector. I think you're going to have a hard time finding an 8 AWG connector if they even exist. Even if it does, the connector itself might be too big to fit snuggly on the bosch terminal. Another thing you could try is use a ring terminal and bolt the ring and the bosch terminal together using the hole in the bosch terminal. It would probably have to be a small bolt and you probably want to heat shrink over that too.

You could use 10 AWG for a 30A load.

...left over from his audio install and would like to use it for my carPC install.

In addition to my carPC, an HD radio module and a tire pressure monitoring system will be hooked up to it.

Seeing that I have a bosch relay sitting in my toolbox, I'm going to incorporate an on/off switch to kill power to everything if I ever need to take it all out. Mostly for the PC for updates and maintainance.

...at a shot.

If you can't find any singles, just cut off some strands at the connector to fit a 10 or 12 ga. and heatshrink it after crimping or soldering.

Don't forget to slip some heat shrink over it after you crimp it.