Product request: Wide 上履き

Request for product recommendation: wide 上履き。

My son has super wide feet, and 上履き in his size, even with their flexible material, won't fit him. We have to buy him sizes too large, to the point that it looks like he is wearing clown shoes.

I have knee problems from a life time of growing up wearing too large of shoes to accommodate my own wide feet, and I don't want to subject my son to the same fate.

Does anyone know a place (online? West Tokyo?) that sells wide 上履き?

by matto89