You'll get confusing answers about Purple sets, since a lot of people refer to them as the Very Rare sets. (Hecatomb, etc,)
An ATO, Winter Event enhancement can be catalyzed which will turn them into a Superior version. Those can only be slotted on level 50s.
Though the Very Rare (purple) are also the only ones able to be slotted on level 50s.
Best ways to get them, varies.
If you have the inf, buying them off the AH.
You can catalyze them from a non-superior. And since catalysts drop one a day for a 50, feasible. (or just buy them from the merit vendor)
Buying Super Packs will get you random ATOs (non-superior) that can be catalyzed.
Spending 100 reward merits at the Merit vendor can also get the non-superior version.