I’d start as close to home as possible. Do you have any sister(s), cousins, close friends of roughly your size, with whom you might trade favours for the odd item of clothing? I’ve been known to pass on clothes that don’t fit or don’t suit to members of my family, and in turn they might lend me something I only need as a one off.
Separately, it would be worthwhile letting anyone relevant know that vouchers for a particular clothing shop would be the most welcome Christmas / birthday gift.
Beyond that - I don’t know what to say about charity shops. They clothed me very stylishly as a student decades ago, but you need to be lucky in your location, and have the time to sift. Hard to say if online pre-owned sites are more worthwhile - but obviously you can try both. Plenty of people nowadays seem to buy everything on Vinted, and don’t feel the need to look elsewhere.
Some supermarkets can be good. Look for simple shapes and natural fibres. I know someone who has been very enthusiastic about Sainsbury’s organic cotton t shirts. Don’t know if they still stock them.
TK Maxx, especially the more expensive Gold Label section, can be wondrously rewarding, if you understand what to ignore. But not always.
If you can stand the multiple emails it’s worth getting on the mailing lists of shops you’d like to buy from - so you’ll know when their sales are, are ‘get your eye in’ as regards new shapes and proportions. (Even minimalist style evolves over time.)
I do think a little knowledge - about fabrics, clothing production, laundry methods, ironing and storage - can help a lot in maintaining a stylish wardrobe, whatever your budget.