Look for tropical hardwoods; the general mahogany family - Iroko, Sapele or whatever the go-to mahogany substitute was for the thing you're buying. Other less mould-prone hardwoods will also do, oak etc. And tbh cheaper (stained) pines and the like will probably be ok as long as they've been treated right. Go for solid timber, not veneer unless you're buying new and the product is designed for bathroom/kitchen.
Gumtree, local groups, ebay, facebook marketplace etc.
Surprisingly, some architectural salvage / house clearance / furniture renovation places can have a good bargain or three.
Sapele is the modern mahogany, Iroko is the poor man's teak - anything you get should be solid wood. If it isn't you will need to seal the surfaces (all of them) with a decent oil-based varnish (use satin if you don't want a high level of shine).
Victoria plum, John Lewis, B&Q or drench would be my starting places. Our stuff came from drenchJust getting the bathroom finished and now i need to start looking for some furniture. Im looking for a tallboy, medium / dark wood, classic styling ( by that i mean not modern) matching wooden wall cabinet and mirror. Any ideas where i should be looking?