The new OSX Lion comes as an upgrade from the Apple App Store – not on a DVD. The good news is that once you’ve paid for it once, Apple allow you to install it on all the computers you own for free! After you purchase it from the App Store you just have to go into the App Store on your other computers and Lion will be there – all paid for and ready to download again!
The Apple page on this is here.
Of course, you need to upgrade all your existing computers to Snow-Leopard first.
The main hitch is that it needs to download lots of files first.
You can make a USB installer and copy it across manually. Here is a way to do this using the built-in Disk Utility in OS X.