blackbat wrote:
I'm sanding down the edges of my stairs as we want a runner carpet
rather than edge-to-edge. Want natural wood at the edges of the
carpets but there are a few gaps to fill.

Any recommendations for flexible wood fillers that can take that that
sort of movement and will also take a stain?

No such animal to my knowledge. There are fillers that take a stain but
unfortunately they look nothing like the same stain on the adjacent wood.
My advice would be to finish a small section of wood first and *then*
find a matching filler. Plastic wood, Brummer stopper or those wax
crayons made by Liberon maybe.