What I do to spawn an ambush is: Go to Lemoyne, in Bayou Nwa, go to the road along the Kamassa River, head north on that road (it will take you along to the left of the Stranger's shack; but as soon as you get past the fork that would take you over the covered bridge that heads to Rhodes, turn your back to the direction you want to go (north) for a minute or two (this will allow the game to spawn the ambush). Then head north along the road and the ambush should spawn (you should see the carriage block your path). If not, travel down the road (north) and back and repeat. You should be able to spawn the ambush.
Hopefully this isn't as confusing as it looks. It really does work for me, though.