Games Workshop Paint Bundles
Games Workshop do not sell many plain paint sets — Wayland list none at all — but they do sell bundles that pair a full army box with the paints to paint it. Those are worth knowing about if you want the pots, because the paint in one is often most of its price: the Getting Started boxes below each carry eleven Citadel pots. Every set here also contains miniatures, which is stated on the card, because judging one on price-per-pot alone would miss most of what you are buying.
Already know the colours you need? Use the paint matcher to find them (or a cheaper equivalent) in any brand.