Damn that is super disheartening to hear. The design was based around that product and finding out it's going to be harder to get means a potential need for a redesign that I don't feel I have the time for.
It seems that Adafruit still sells the PCB which is linked below:
I thought I found more stock of the stick at Robotshop.com but they seem to be out as well. Looks like the a-stick is gone :(
There may be good news however. Looking at the PCB Adafruit sells it looks like the no longer available a-stick had a PCB-tail layout that matched existing larger a-sticks. If this is true it may be possible to replace the mini stick with one similar to the stick below:
One problem is I can't seem to find a stick without the attached button. Worse comes to worse the buttons pins could be cut off. If the new stick matches the PCB then it can hopefully be fit through the existing faceplate as well. From here the only redesign necessary would be to make a new cover for the stick. The a-stick would also protrude out further.
Hope this helps.