As a patient at any clinic, you have the right to use any pharmacy of your choosing. Just ask your doctor to send future prescriptions wherever you like. They can refuse if there are legitimate reasons for example something that the pharmacy doesn't offer, but otherwise it's your entitlement.

As for Grow stock. It's nothing to do with Specials. They don't hold it in stock. It has to be ordered in on demand from the supplier which will delay things a few days. All pharmacies are like this with all stock for any brands that aren't their own. Grow only had one single Grow strain in stock until this morning. That was the case no matter what pharmacy you go with.

Just remember, as someone else already said here. The prices may vary as certain discounts, subsidies and other schemes are sometimes limited only to specific pharmacies. Likewise with delivery fees.