As Mike suggested, the first place you can look is Toshiba. Honestly we do not really order from them very often because they don't carry a lot of discs anymore. If you cant find a disk from a friend, who has the same computer as you, which would be extremely rare, then you should look for a third party to buy them from:
http/www.recovery-disks.com/toshiba-satellite-u500-1dr.html
This is the disc set for your particular machine. The
Toshiba Satellite U500-1DR recovery disk set will install the operating system, drivers and utilities for your computer. Use this disc if you need to wipe out a virus or malware on your computer or you installed a new hard drive into the computer.
The website above is what we use to buy recovery disks for customers when they need to purchase discs to restore their machines back to factory, or we do hard drive replacements for them.
Let us know if you have any questions or run into any trouble utilizing the recovery discs.