- where can I buy dividers, compass and charts (e.g. St. Croix river
area),

St Crois River? Best I can say is, go downstream until you hit a bridge, turn
right and that is Minnesota, turn left and it is Wisconsin.

seriously, if you really want a genuine nav course, go to any small airport
that gives flying lessons and sign for a nav course. you won't learn celestial
nav, because that went the way of the dodo bird in aviation about 60 years ago,
and in seagoing about 20 years ago. Sorta like button hooks and boots.