Have you tried Boots Prescription In Stock system? Online.
It shows where you can obtain drugs within x miles of your postcode. To search a wider area, you'd need to put in a postcode slightly outside your own.
At the moment, when there is a shortage, it's better if possible to have a physical paper prescription that you can take to any pharmacy, rather than having only one pharmacy that you nominate.
The alternatives to Oestrogel are Sandrena gel, Lenzetto (spray) or an estrogen-only patch.
Pharmacists were given the legal powers to offer suitable alternatives, when the shortages became widespread, so you ought to go back to them and ask, first.
There are also several menopause websites that have tables of HRT and the alternatives/ doses. I think one is the Women's Health Concern, and the BMS and the site of Dr Newson.