Hi Navera,
For clothes, there are three places you can go:
1. Shamshuipo Street Market - you can get bibs, sleepers, onesizes etc. Prices range from $20 - 50. Take the Fuk Wa Street exit at the Sham Shui Po MTR station.
2. Stanley - Has got the cutest baby things. You'll find all the brands, Baby Gap, Osh Kosh, Tommy H etc. All export overuns.
Prices also range from $20 -$100
3. Fa Yuen Street (Prince Edward MTR Station) - Also have export over runs at attractive prices.
(Avoid all places on weekends)
For all the other items, there's a tiny store next to the Tsan Yuk Hospital that I've bought stuff from
Ah Chi Co
Also have a look at the Trading Post in the SCMP on Sunday for expat sales for strollers, etc.