I got my maternity clothes from Shopee and JumpEatCry.
Shopee is a more affordable option but sizing wise best to look at people's reviews before purchasing.
JumpEatCry's clothes are super comfortable and can be worn during maternity, nursing and even as normal wear. Slightly more expensive but I wait till their sales.
You might want to check out Uniqlo as well. I did not get any from there but their maternity clothes does not seem expensive.
Reyne