Hi, Adafruit is my only distributor at this time. I’m not selling them direct.
You may be able to find some stock of the old SmartMatrix Shield V3 (Kit) from Adafruit’s distributors. There’s at least a partial list of distributors that have purchased SmartMatrix Shields in the past at the bottom of the Adafruit product page. Cool Components (in the UK) had stock last time I checked.
Also, you don’t need the shield to use the SmartMatrix Library. There are instructions here on how to wire something up yourself.
Update 2019-03-22: I noticed some stock of SmartMatrix Shield V3 at Digikey