This isn't the "quick" nor the healthy answer, but I personally do a twice-cooked system. I leave the juice from the can in the mix, and boil it down, which takes 15 minutes or so. Retaining the flavor of the canned fluid is essential, in my opinion.
Then I "fry" the dry-ish mixture which is beginning to separate in maybe 1 tbsp preheated lard per can of beans, stirring a lot at first. Lastly, very slightly simmer this on low heat for 1 hour or so, stirring occasionally. Flavor is awesome. Any kinds of beans can be prepared in this way, and it is similar (varying on oil and legume variety) to other paste concoctions created elsewhere.. such as in the middle east.
Note that this method negates the need for lots of "mashing". So, if you're more looking for less effort, than necessarily for "quick".. this could be your answer :)
There are many different kinds of "an" paste. Left unspecified, the generic type is "red beans", specifically azuki. I've found it pretty easy to find azuki beans in Germany, the US, Japan and Korea, so I can't imagine it being terribly hard anywhere else; in the US and Germany it was often sold by natural foods shops when there wasn't an Asian market nearby. The name was sometimes spelled adzuki.
One reasonable alternative is white navy beans, sometimes called cannellini beans in the US, or perhaps lima beans. Either one can be used to make "shiro-an", or white an. Mixed with salted cherry blossoms, shiro-an becomes sakura-an. Alternatively, it could potentially be flavored with sesame or something like the herb yomogi.
Another alternative is uguisu-an, which, as far as I understand, is made from green peas and generally has an improbably bright green color. The word comes from the Japanese word for nightingale, supposedly due to the visual similarity to nightingale poo. In the Japanese sources I have checked, it's not 100% clear to me whether one should be using dried or fresh/frozen shelled peas. It looks like there are precedents for both.
Fresh green soybeans (or frozen ones), known as edamame in Japanese, can be used to make zunda, which is from Sendai. I don't know how well zunda would stand up to baking, though, as I find zunda better suited for serving with mochi or shiratama dumplings.
In China, lotus seed paste, made from boiled ginko nuts and sugar, is also popular, and could be used in baked goods.
Best Answer
Black bean paste is also incorporated into Asian cuisine, and has a similar flavor, although earthier. Add a little sugar, maybe a little ground cashew or peanuts for nuttiness. It won't be the same, but it should work well enough.
Since black beans are more common, this may work well enough for you.