I buy all the kids’ clothes on eBay. I always search for "lots" which are large lots of clothes – they are usually the best deal. For example, a mother might sell a "lot" of 4 year old clothes – all of her kid’s 4 year old clothes when he outgrows them. Another way to find good deals is to use the advanced search option to search in the kids’ clothes category for all lots that have zero bids. Usually those are lots that have bad pictures, bad titles or bad descriptions but they might still be good clothes.
I also bought lots of maternity clothes on eBay when I was pregnant.
Also, don’t forget eSnipe when you go to do your bidding!