Directly from the PMK website:
As production time allows, we plan to keep the following sets in stock indefinitely:
Eve, Midnight, Deep Space, Green Tea, Dolch, Otaku, Quartz, Think Different, Jukebox, Hana, Granite, Stealth, Ice Cap, Industrial and Borealis.
Please do not email Melissa to find out when additional sets will be available. She will no longer be answering these emails. We will announce, either through an email or in "What's New", when sets will be available prior to putting them in inventory.
Looks like Borealis will be restocked in the future.