So far I've found examples from the late 14th century with this poem that doesn't have much historical basis just romanticism (https://en.wikipedia.org/wiki/Sir_Gawain_and_the_Green_Knight). And there's this book for the last knight-errant http://www.amazon.com/The-Last-Knight-Errant-Woodville/dp/1848851499. But these periods are after what I'm looking for, that is from the Early Medieval Period to the High Medieval Period.
I don't think there are any examples to support the view that knight-errant really existed other than as a part-time position for competitive jousting/tourney groups/participants and professional duelists of the noble class.
I have also found the closest parallel to ronin banditry in this source from my reddit searches: https://en.wikipedia.org/wiki/Robber_baron#Interregnum_period.
On the other side of the world ronin had many instances and flavors in terms of what they did an depending on what feudal period. I just can't find that level of variation and sources in the west.
You could look to the Scottish knights errant involved in the 1330 Battle of Teba, led by Sir James Douglas.
Disclaimer
My PhD was in medieval Scottish Literature, so my source here is John Barbour. The fidelity of the information is therefore questionable. But still, perhaps a route to follow.