On the old Illinois frontier of 1700 AD, I expect much of the Jesuit missionaries' time was spent outdoors, trawling for prospective converts, so the wide-brimmed, short-crowned cappello romano seems the obvious choice (think Father Brown, or "Black Robe"). However, if anyone has a line on a biretta - the 3-finned cap with a pompom in the middle - I'd be interested in that, as well. You'd think the cappello should be a fairly easy thing to find - it's really just a basic hat shape popular with farmers and the like through the centuries. So far I've had no luck: there have been hints of a download here on MTS by someone named bettye, but I can't seem to locate it. As long as it's black with a wide, flat brim and a rounded top, it should work.
I've also posted this query on medievalsims, BTW.