I used to be able to download temple run onto my iPod 2nd generation. I then had to restore the iPod but now I can't re-download temple run onto it. It says you need a higher ios number. Is there anything I can do to get it back?

So, you need an older version of the iOS app, because the current version no longer works on your iPod touch.

If you do regular backups of your hard drive (as you should), such as by using Apple's built-in Time Machine (if you use a Mac), you may find the older version of the app in the backup archive, and restore it (replacing the newer version). They are stored in the iTunes Media folder, in the Mobile Applications sub-folder. You should then be able to sync the older version to your iPod touch.

I can't think of any other way to get it back...

NOTE: An 2nd gen iPod touch can run up to iOS 4.2.1. If your iPod touch is running something earlier, and that is why the app no longer works, you can also consider updating the iPod's iOS.

I use an even older iPhone (original). I avoid this potential problem by aways updating its apps on the iPhone, not in iTunes on the computer. When I do it on the iPhone, it will not download a newer version of an app that won't run on my old iPhone.

I believe it is in your user's Music folder, by default. So something like

C:\Users\[your user name]\Music\iTunes\iTunes Media

If you open iTunes preferences and go to the Advanced tab, your location setting should be shown there.