I have recently replaced a number of flags on some of my islamic units with better ones. These were produced by a chap called Pete Barfield (he posted them on Twitter and Facebook) who kindly is happy for people to use and modify as they wish. He made them for 28mm but a bit of resizing sorts that for 15mm. Some of these are as Pete made, but most have had additional images added over his base colour flag.
(https://i.imgur.com/ESP8wVM.jpg)
(https://i.imgur.com/hRuI6pl.jpg)
(https://i.imgur.com/pFTr3xq.jpg)
(https://i.imgur.com/fHw1kUn.jpg)
(https://i.imgur.com/6a01urW.jpg)
(https://i.imgur.com/FsrSrG1.jpg)
(https://i.imgur.com/9htJLp1.jpg)