But if you find yourself with some citrus (lemon, lime, orange, grapefruit, etc) as well as some eggs, butter and sugar, and a few minutes to kill, you can make your own...
Now I've run across many recipes, some claiming to be 'foolproof', others saying they're difficult but worth the trouble. I've been using the basic recipe below since I first tried making the stuff, and have never once had any difficulty with it. You can go the traditional lemon route, or you can use other citrus flavors, or even a combination. The most recent batch I've made was with a lemon, lime and orange I found languishing in the fridge. It was either that or sangria, but I can't spread sangria on my toast in the morning (but I wish I could! ;).
A double-boiler will be useful in this recipe, but not strictly necessary. You can make do with a heat-proof bowl, such as Pyrex, and a saucepan whose mouth is slightly smaller than the bowl, allowing it to sit on top without falling in. You will be simmering water in the pan to heat the contents of the bowl, and since we are working with eggs you must make sure the water does *not* touch the bottom of the bowl you use! Otherwise you will have lemon-y scrambled eggs. (If that's confusing, check the article here, it might help ;) Other than that, the only thing you need is a little patience as this is one recipe that cannot be left alone on the stove; luckily it only takes a few minutes to make. So let the dog out in the back, tell your significant other to answer the phone, and give it a try:
5 egg yolks
1 cup sugar
1/3 cup lemon juice (2-3 lemons)
1 stick butter, room temp and cut into pieces
lemon zest (optional*)
Combine egg yolks and sugar together until smooth. Add in lemon juice (and zest if you are using it) and place mixture in top of the double boiler over medium heat. Grab a whisk and start whisking... it will take about 10 minutes, so don't worry about whisking quickly, you can go at a comfortable pace. Just keep the mixture moving so as to avoid it cooking faster on the bottom. After a few minutes, you will start to see it thicken... this is good. You can go more or less than the 10 minute mark, depending on how thick you want it, just keep whisking until you are ready to take it off the heat. Once it is thickened, remove the from the heat and drop in the butter, one or two pieces at a time and whisk it in as it melts. Do this with all the butter, making sure it is completely melted and there are no lumps. Let the curd cool slightly then put in an air-tight container (I use my trusty canning jars) and store in the fridge. Break out the crumpets and scones and enjoy! ;)
*Note on using zest: I hate to let food go to waste, so always zest my citrus before juicing it so the zest can be used as well. The first few times I added it in to the curd knowing it would add an extra kick of citrus flavor, and it does. Only thing is, I found the little bits of zest made the finished product seem a bit gritty, rather than smooth and velvety. So now I put the zest in the juice and give it a blend with an immersion blender to break down the zest before adding it to the curd. I still get lovely little flecks of citrus flavor, but no chewy bits!