If you're using a G3, you can't.
If you're using a G4 or G5, purchase Mac OS X 10.5 on DVD.
If you're using an Intel Mac, purchase Mac OS X 10.6 on DVD, or get it from Apple for free if you're a paid MobileMe member.
Intel Macs with a Core 2 Duo or better CPU can run Lion. This is available from the Mac App Store in 10.6.6 and newer or on an USB flash drive.
Snow Leopard/10.6.x Requirements...
* Mac computer with an Intel processor
* 1GB of memory (I say 2GB at least)
* 5GB of available disk space (I say 30GB at least)
* DVD drive for installation
* Some features require a compatible Internet service provider; fees may apply.
* Some features require Apple’s MobileMe service; fees and terms apply.
Which apps work with Mac OS X 10.6?...
It looks like they do still have it:
And it's been reported that if you have a MobileMe account Apple will send one free.
If it's a core Duo & not a Core2Duo, then it'll only run in 32 bit mode.