I sometimes buy from charity shops. Not quite clothing, but I once got a beautiful, as new, Hotter leather bag from our local shop for £1. A bargain! I do find, though, that some of the prices of high street charity shops are not much lower than the new prices in shops. I have seen items from places like Primark labelled at higher prices than they were originally sold for new.
In general I find charity shops are a good place to buy quality items cheaply, but not cheap things very cheaply. It also pays to look in little places outside the city centre.
For vintage things, it is worth looking around places like a 1940’s rally and similar. I have found period clothing on sale fairly cheaply and on close inspection some of it is fairly well made (recently though). It is also possible to get material and accessories from the period. Sometimes I have found things in antique shops as well (I have a lace choker I made in which I used antique Victorian shoe buttons, bought very cheaply, for the fastening).