Stores that sell affordable maternity wear : H&M, Love Bonito, Jumpeatcry, dearcollective. Uniqlo used to sell too but I'm not sure if they still do.
Can also consider getting tent dresses that can accommodate the growing belly (sleeveless type so still easy to just bring down the sleeves for nursing/pumping subsequently). That's what I did cos I wanted clothes with longer mileage, beyond pregnancy and nursing.