I think you can take a treated 2x4 and cut new risers. I even made handles from treated 2x2s.If there is enough of the old riser left, just lay it on a treated 2x4, mark it and cut it. HTHS

Internet search show that Ace Hardware has some that sell for $4.49 each plus s/h. Suggest calling local stores. Ace calls the part - wheelbarrow wedge.

Unfortunately the current riser is badly rotted so can't make a duplicate.
I'll check with Ace Hardware here and see if they have the part.

Thanks for posting.
Julia