As promised, here is the open prescribing site:
https://openprescribing.net/analyse/
You can search the data by surgery and product, or by CCG and product. If you search by surgery then product, it will compare your surgery with other practices in your CCG, identifying which surgeries prescribed Fiasp, in your case, and how many prescriptions of it they prescribed in July '19, which is the latest available data.
The data shows, graphically where there are peaks in prescribing of a product. You could then look up where they are, and target surrounding pharmacies.
Not all pharmacies keep "office hours". The pharmacy in our village is open until 10pm, Monday - Friday, something like 8pm on Saturdays and 6pm Sundays, and they are incredibly accommodating.
If all else fails, you could call the pharmacy at the hospital where your clinic is? If the consultants there prescribe it, they're likely to supply it?