A lot of the basic supplies you need (flour, sugar, etc) are available in food stores. HomePlus, in particular, also has a small line of cake tins, cupcake trays, etc. But you can’t find everything… so here are a few other places for those bits you can’t find.
By Metro: Line 1 (Dark Blue) to Jongno-5 ga Station, Exit 7. Walk straight on Dongho-ro, taking the brick bridge across the Cheonggyecheon stream. As you cross the stream, you’ll see the entrance to the market to the right, on the cross-street (running along the stream), 방산종 합시장. Turn in there, and then I suggest taking the first alley on the left. There’s probably more than just the bit I’ve highlighted on the map.
Small but helpful store (particularly if you live in or close to Seongbuk-dong / Hansung University Metro station, as I do). Basically a small neighborhood shop specializing in foreign products. E.g., my most convenient store for confectioners sugar, dried coconut, etc. Not sure I’d recommend a special trip, but if you’re in the neighborhood for some reason, it’s worth checking out.
By Metro: For what it is, I would not bother going there by metro.
It’s just a 2-minute walk downhill from the Gilsangsa Shrine.