You're probably best trying Boots or a similar big chain and ask them to give you a note saying they can't get it in, so you can show your doctor the note and get an altered prescription.
I had a prescription for micronised progesterone 200mg pessaries for days. I tried various pharmacies and they said they stock 100s, not 200s.
I was even told it isn't available in the UK by one pharmacist.
In Boots they said they had 100s, I finally gave up, got my other items and asked for a note for my doctor so I could get a different prescription for 24 of 100s instead of the 12 of 200s on the prescription.
The guy printed me off an owing slip for the 12 x 200mg pessaries, this was my note for the doctor.
The next day I got a text and my 200mg Utrogestan with vaginal applicators was waiting for me to collect in Boots.
Someone somewhere will be able to fill your prescription for you.
As I found, once they actually take your prescription and give you an owing slip, miracles can happen