Op shops and end of season sales as the kids get older and op shops don't have as much.
My son (2) is almost exclusively dressed in op shop clothing, with a few other thrown in where I can't get enough at the op shop. My older two (girls-7 and 9) have a fair bit of op shop stuff, plus hand-me-downs and a few things I get at end of season sales.
I get comments all the time about how nicely dressed they are, which I think is because their clothing is a bit more unique as its from all over the place. People are always shocked when I show them how much is from op shops!
I buy bigger sizes if I find something really good too and just put it away til it fits.