Mountain Lion is compatible only with Early 2008 or newer Mac Pros. Lion may still be available if you call the online Apple Store, though a couple of recent threads have reported it as no longer available. If the Apple Store still has it, they will ask for your Apple ID and credit card number and will then send you a code which you can redeem in the Mac App Store.
Alternatively, you may still be able to find the USB installer at a local Apple Store or an Apple dealer, though available stocks will probably disappear quickly if they have not already. Otherwise you'll have to look for the USB installer on the open market, such as through eBay or a Mac User Group. Those USB installers are starting to rise in price.