It's not sold out, as it's not yet launched - as per L3+ "Notes" - bullet #6:
"Bitmain recommends the new PSU that is especially designed for the Antminer L3+ and will be available to purchase after 20 June 2017 with shipping start date before that of this Antminer L3+ batch."
Thanks for the clarifications. I have edited my answer too regarding the aternatives : an EVGA supernova 10 50 GS seems to be a good option.