SUM does carry them occasionally, I think I have seen them have it twice in the past 10 or so months. I bought one, but it lasted only a couple of weeks and died presumable when it got caught in the overflow.
The best way to know if they have any is to check weekly their in-stock PDF which is included on their specials page. BTW, they are not inexpensive for such a small fish, they charge $25 for them.
I have one and looked everywhere for at least 6 months. The tail spot is a great addition with tonnes of personality. SUM gets them from time to time. I actually got mine from BA Hamilton and I've also seen them at BA north York. But best to get from SUM.
Drove for an hour to BA in Hamilton and finally got the tail spot blenny. Paid $35 for the little guy a bit pricey I think. I expected him to be very shy which he was at first but now hes out and about and grazing on the live rock. Hes like the coolest little nano fish.