Hard to find in HK. H&M has a small selection, so does Mothercare. A friend placed an order at Old Navy for most of her larger sized maternity wear and had it shipped to HK.
Everyone is different and I suppose it would depend if you are uncomfortable. I never did wear a support or maternity bra, although my regular bra size went up one size I just wore a larger size of regular bra. Some people really dislike underwire at some stage of their pregnancy, but it never bothered me. I also have friends who switched to stretchy bras without underwire and sports bras because they were more comfortable for them, but I found sports bras in particular to be too hot in the HK summer when I was expecting.
Marks n Spencer UK online shopping will ship to HK. I try things on in the HK shop and then order it online for half the price. Delivery is HK$160 irrespective of whether you buy 1 thing or 25 things. I also bought towels and baking stuff.
I got some of my maternity clothes from Sogo (there's a little corner at the children's section that sells maternity clothes, bought jeans and pants there) and this place called linea Negra (at central opposite landmark above diesel shop), also got some maternity bras at Marks n Spencer hope this helps.
I got a lot of stuff (larger sizes) at Fa Yuen Street in Mongkok. You have to really look through the stores that are on the sides of the market but it is worth it as the prices are so much cheaper than the stores in Central.