I am 77, 5ft 2in and size 10/12. I buy clothes in Sainsburies, M&CoLands End, who do a good line in shirts and, blouses. David Nieper is probably your style, but expensive.
Availability of clothes, does rather depend on how your taste runs, but if it is what would previously be describe as 'classic' styles, I think the problem may well be that now so many older women dress in a similar style to younger women, there is less demand for clothes for that style of dressing, so the usual chain stores are no longer stocking them.
Your size shouldn't be a problem, I have seen no lack of smaller sizes on the racks in all the shops I use.