When I was painting some Janissaries last year, a friend who's quite keen on the Ottomans posted:
Janissary trivs: the uniform items were shoes (colour-coded to social status, Janissaries entitled to red leather) trousers (specific to regiment) and headdress.
Coats were private purchase, for some reason.
We can expect a degree of uniformity due to coming from the same sources, local availability of fabric, and not wanting to look different...
No idea on the source though... And how it may have differed on campaign...