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I have bought individual bottles from Amazon without problem. It looks like some of them are "add on" only, but it's worth checking for the colours you're looking for.
Scale Model shop is somewhere else to try - https://www.scalemodelshop.co.uk/products/paints/vallejo-paints
Complete senior moment on my part, sorry about that .
On topic, I second the Historex recommendation. I didn't realise andrea did individual bottles, I think I've only ever seen sets. I'll need to have a look myself and see what's out there