I've gotten them from Safeway before.
In the past I've had problems with the grit in the kalk solution plugging up the needle. So lately I have been going to the opposite end of the the PH scale and injecting aiptasia with vinegar instead. So far it has worked more effectively for me than Kalk.
Disclaimer:
Since it is an acid you want to watch how much you use at one time and make sure your alk is good. So far I have only used this method in larger 130+ gallon tanks. YMMV.