They are specific to the device so you can't use someone else's. It's too late to save SHSHs after the signing window closes. However, you may be able to save an SHSH for the firmware version you are currently running, even though the signing window has already closed, with iFaith. It's only available for Windows at this time.
If you have an iPod Touch 2G, an SHSH is not required. You do need a saved SHSH to restore to stock 4.x firmware, but you do not need it to restore to custom 4.x firmware. An SHSH is required for all stock and custom firmware restores on old firmware on the iPod Touch 3G and iPod Touch 4G.