In the UK, almost all garden centres and nurseries stock a range of Kniphofia. With that being said, unfortunately there has been significant homogenisation in recent years between what garden centres offer, with the newer Popsicle, Poco, Glow and Pyromania series, despite their merits, displacing many classic cultivars. The following list are nurseries which I have found to offer a good range of such older/rarer cultivars alongside newer introductions they have bred themselves:
Barnes Nursery – particularly good for new American Introductions
Plantbase – Particularly good for species Kniphofia