All you need is two ingredients and two hours
I never have condensed milk in my pantry when I actually need it, and that's why I always find myself scrambling for a condensed milk alternative when a recipe calls for the stuff. But in order to find the best condensed milk substitute, you have to first understand what condensed milk is. Yes, condensed milk usually comes in a can, and yes, it's actually milk—but it's milk that's been heated until it loses about half of its water content. This is why condensed milk has a thick, creamy consistency rather than a liquid one. And when we talk about condensed milk, we're usually talking about sweetened condensed milk, which has sugar added. This addition of sugar is what makes condensed milk different from evaporated milk, which is unsweetened though made using the same heating and evaporation process.
As Harold McGee explains in On Food and Cooking, "For sweetened condensed milk, the milk is first concentrated by evaporation, and then table sugar is added to give a total sugar concentration of about 55 percent." So if you're looking for an easy condensed milk substitute, you can simply add sugar to evaporated milk so it has the correct sugar content. Folks online recommend adding about 1¼ cups of white sugar to 1 cup of evaporated milk.
But I'm willing to bet that if you don't have cans of sweetened condensed milk just sitting in your pantry already, you probably don't have cans of evaporated milk either. Fortunately, it's actually quite simple to make condensed milk at home. As Stephanie Le writes for Food52, all you need to make homemade sweetened condensed milk is 1½ cups of whole milk, ½ cup of granulated sugar, and about two hours. Simply mix the two ingredients in a heavy-bottomed saucepan, and let them simmer down. Le does note, "This homemade version won't be nearly as thick as canned sweetened condensed milk," but adds, "it does have the same sweet, milky flavor."
If you're looking for a dairy-free sweetened condensed milk substitute, try a sweetened condensed coconut milk. You can also make this vegan condensed milk substitute at home, by boiling down full-fat coconut milk with sugar.
Or you could always just stock your pantry with cans of condensed milk, just in case. But making condensed milk at home is easy—and delicious—enough that I wouldn't blame you if you "forgot" to pick up condensed milk every time you went to the store.