What is used in the production of dubu (Korean tofu)?
As far as I know, Korean and Japanese tofu are effectively identical in taste and preparation: my Japanese wife, who is generally quite particular about her ingredients, happily buys Korean brands from a Korean grocery for her Japanese cooking.

If you're dead set on making your own, in both countries the key ingredient for curdling is nigari (Japanese) aka 간수 gansu (Korean), basically magnesium chloride.  Epsom salts are magnesium sulfate, so close but not the same thing; lemon juice is out of the question (for one thing, neither countries had lemons until recently!).

When experimenting, bear in mind that both countries use several different styles of tofu.  "Standard" tofu/dubu is momen-dofu in Japan, but Korean soft tofu (sundubu) is even softer than Japanese kinugoshi-tofu and more akin to a rare Japanese type called oboro-tofu.