ROTJ Luke Skywalker does not have jack boots - but rather officer/riding boots that are a little higher.
I think the authentic boots were indeed German officer boots that were the same as the grunts' jack boots except for the height.
He wore the shorter boots only as a pilot in ANH. (and perhaps in ESB inside Echo base... but I have not been able to tell)
The Royal Guards in ROTJ and ROTS should have red boots, but the 501st accepts black jack boots because red boots being very hard to find.
The CRL for AOTC accepts black only, though.
The German boots should be available, in different versions, in men's sizes from WWII Army Surplus and army antiques stores. They were made in large numbers also for decades after WWII.
But if you do find some more comfortable boots... I would advice that you DO go for comfort over absolute accuracy.
They need only be "authentic enough" for legion approval. You don't want to cut down on your trooping because your feet hurt...
