I was going to say it depends on your budget, but to be honest babies and toddlers grow so quickly that unless you're committed to selling them on ebay later, there's no point in forking out loads on new baby clothes.
Asda and Tesco are a great place to start - their designs are based on what Next and Gap are bringing out the same season.
Primark is also very good quality, dirt cheap and about as fairly traded as you're going to get on the high street cos most if not all of their stuff is made in Portugal rather than a sweat shop somewhere.
H&M were great for my ds, but I've not been there in the last couple of years.
Do a Google for sites that offer new and nearly new baby clothes. Some sellers will buy up huge swathes of brand new stock from designer shops closing down and sell them cheap online. Other site sell second hand using a star system so you know how worn the clothes are. I've used Smile By Post loads of times and got some alsot brand new Monsoon, Next and Zara stuff.
Mackays has also just had a revamp and seems to have much nicer childrens and babywear - tht mighht be a regional store though?
Vertbaudet are also good quality, but agina very slim fitting.