You can make tsubushi-an or tsubu-an (anko) surprisingly easily. To make it with the level of sweetness that you prefer, it's best to make it yourself. Even if the beans fall apart a bit don't worry about it.
Mizu-ame (starch based sweet syrup; use sugar syrup if unavailable)
2 tablespoons if you have it
Rinse the beans, add 3 times the amount of beans in water, and start cooking.
Set the heat to medium and bring to a boil. 4 to 5 minutes after it reaches boil, drain the beans into a colander. Run water over the beans to get rid of the scum. Put back in the pot with 3 times the amount of ater again.
Repeat step 2 (bring to a boil, drain and rinse) 3 times, then put the beans back in the pot with 2 times the amount of water. When it comes to a boil turn the heat down to low, and simmer until the beans are tender (about 30 minutes).
Add 1/2 the amount of sugar to the pot, and shake the pot to dissolve the sugar. Add the rest of the sugar and dissolve. Turn off the heat, and leave the pot so that the beans can absorb more moisture.
Turn the heat back on. When it starts to bubble, turn the heat down to low and simmer while stirring gently with a wooden spatula.
If you can see the bottom of the pot if you scrape it with the spatula, add the salt and the beans are done. If the beans are a bit liquid when hot, they will stiffen when cool.
Story Behind this Recipe
Sometimes I get an irresistable urge to eat anko. If you make a lot you can turn it into all kinds of snacks, so I keep it stocked in the freezer.
In order to prevent the beans from popping out of the water while they are simmering, add some "shock water" (1/2 to 1 cup, 100-200 ml) halfway through. If you put kitchen towel right on top of the simmering beans in step 3 to act as an otoshibuta or drop lid, and cook the beans gently with love, the beans are much less likely to fall apart. If you add 250 g of sugar, it will be as sweet as the usual storebought anko.