I buy them from all over - Sainsbury's, Tesco, Asda, Lidl, Aldi, M&S, C&A, H&M, Zara, Boden, JojoMamanBebe, John Lewis, M&Co, Boots, Debenhams, Gap, Vertbaudet, La Redoute, Matalan, second hand... My children are only young though.
I am happy to buy higher quality, more expensive stuff, especially now I have two children, as things can be handed down. I tend to buy stuff with a bit of growing room so it will last longer. I wouldn't splash out on something because it was "designer" (and an obvious name splashed across the front would put me right off - I refuse to buy clothes with writing on
), but I would pay more for something well-made out of nice, good quality fabric.
But having said that, I have had some supermarket cheap stuff that has lasted and lasted and looked like new.