Acorn Fabrics here in Lancashire are specialists in shirting fabrics . Not the fun printed poplins you're using, but classic shirting of all sorts from cosy brushed cottons to spun silk, all the interlinings, collar stays and pearl or shell buttons you could want. Made my SO several shirts which he reckoned were as good as those from a professional custom shirtmaker and better than anything he could buy in the shops; the 'proper' fabrics and other bits'n'bobs made a huge difference. Have also used fabric from them for clothing for myself.
No connection to the company other than as a long-term happy customer and know that finding the right fabrics for specialist purposes can be a problem.