You can buy walking shoes in every shoe shops in old quarter.
In this season, it is dry in Mai Chau so I think there’s no need of buying a professional walking pair of shoes, just take a normal one.
Last time I bought my walking shoes at UMOVE 74 Pham Huy Thong and 111 Lang Ha
I also see waterproof shoes are sold in Le Duan Street (for about 30$).