I tend to use ebay. Browse to the baby clothes section, then use the criteria on the left (size, gender etc) and click "choose more" on these. You can then specify or exclude certain brands, types of clothing, colours, materials etc. Leave the "unspecified" box unticked if you want more streamlined results, but tick it if you're happy to search through a lot (and I mean a LOT) more crap for a bargain.
Also try searching a particular brand, or searching something like "rainbow" or "flower" or "toadstool" or whatever design you want, with the only filter as the age/size and sex. Mini Mode for example had lovely clothes for girls and boys but they're gone now, but plenty on ebay.
Add everything you like to your watch list, until you get bored. Then sort watch list by seller ID and go through and see what each seller has as well as the ones you're already watching. Any you like, add to list as well. Then go through the watch list, weed out anything you're watching which is the only thing that seller has (unless you REALLY like it) and anything you've looked at among all the rest and thought "Meh" and anything above your maximum price per item. As a rule of thumb on ebay I don't pay more than £4 for a t-shirt or top, and no more than £8 for a pair of trousers or a jumper/cardigan/jacket type thing. But your limits will have to be higher if you want Boden stuff - I rarely if ever win this at these prices.
Remember to factor postage cost into this. The reason you are looking at getting lots of stuff from each seller is to cut down on postage, but not all sellers combine postage and if they do, not all of them take a decent amount off. So for example if I was looking at a t shirt which had £2 postage I'd only bid up to £2. This stops you getting carried away, though it's still easy to spend a lot! Just recently I spent over £50 on clothes for DS but realistically he's got most of a wardrobe for the next year or so there.
Keep stuff in your watch list and don't make bids until there is a day or so left. Or if you know you're going to be away from the computer or unlikely to check it for a couple of days then that far in advance is okay. Be aware if you're bidding on quite well-liked or expensive brands (Boden, designer labels, some Next stuff, some Gap) you can get into a bidding war if you bid too early.