Clotted cream (sometimes called clouted cream or Devonshire cream) is a thick cream made by indirectly heating full-cream cow's milk using steam or a water bath and then leaving it in shallow pans to cool slowly. During this time, the cream content rises to the surface and forms 'clots' or 'clouts'. It forms an essential part of a cream tea.There are many make-your-own recipes to be found online, with as many commenters that dispute the resulting taste from such trials. The website europeancuisines.com offers a simple (two ingredients: fresh milk and heavy cream) though time-consuming (24 hours to sit, 1.5 ++ hours to cook) recipe that should taste like the real thing. Even if you're not planning to make your own, the article linked above still makes for an informative and entertaining read.
Another recipe comes from Sustainable Table, and also takes a long while (8-10 hours in the oven, then refrigerate overnight) but won't require you to watch a pot nor fiddle with water temperatures in a double boiler.
For the rest of us, there's Amazon and 6 oz bottles of English Clotted Cream that comes 2 per pack for $12.60 plus shipping.