You'd need to contact Toshiba for their recovery software (either disk or ISO) - anything linked elsewhere is likely to be full of viruses etc.
You should be able to use 'standard' 8.1 installation media, since your license key should be embedded in the BIOS - but Microsoft will likely direct you to Toshiba anyway. If you have access to or can acquire a genuine W8.1 USB or DVD, you could use that