It can be purchased through the VBlank Entertainment website.
The package contains the full PS Vita release on a 2GB Vita card (the game takes up 19.8 MB), it’s region-free, and will run you $30. It should ship to you within 3-4 weeks.
It can install alongside the digital version if you already have it, and is 100% compatible with the existing Cross Save feature on PS4, PS3 and the digital PS Vita version.
Back in November, the game saw a limited retail release on PS4 Blu-ray which is sold out per the retail page linked above.