From November 29th to December 31st, you'll be able to nab all 47 legacy skins in the League of Legends store. If you want all of them at once, there will be a 60% off discount in the form of a Mega Legacy Bundle, which adjusts its price based on what you already own.
Note that in two days time each skin will be on sale for one day at a time for 50% off, so check the schedule if you want one in particular. Just note that there will be no refunds for people who buy a skin and want the discounted price later.
This re-introduction of limited edition skins has caused a minor controversy among fans, who purchased them originally under the pretense that they'd never return to the marketplace, and hence, aren't actually "limited." I'm kind of on the fence myself having bought a few of these, but at the end of the day, they're just skins, so I'm not too worried.
The Legacy Vault [League of Legends]