Okay I've only gotten a handful of kpop merch so I dont know too much. Anyways I bought all of my merch so far at Aliexpress. Let me start off by saying that I was extremely cautious with this site. I looked up reviews about it and they weren't to good but I went against the reviews and was honestly pleasantly surprised with my purchases. Also the site hosts many different people selling products they could be selling the same thing but for different prices or one shop can be more reliable than another etc. The shops are similar to the shops on Amazon and eBay.
They have a variety of things available. Like kpop jewelry, shirts, sweaters, backpacks, mouth masks and a lot more. Many different groups are listed. Depending on what shop you buy it from you can get things for a really good price and most of the time shipping is free. Most of the shops are in China so shipping is lengthy but I haven't had to wait that long(2 weeks at the most) Also they use Asian sizes so I recommend buying a size or 2 higher than your usual american size like I do. Unless you're small/skinny.
So far these are reliable shops (in my opinion) that I have bought Kpop merch from and gotten what I payed for are
At this specific shop you can buy shirts, sweaters, jewelry, backpacks etc. They sell a lot of Exo and BTS stuff they also have Got7 and Block B things plus more. In the other pictures those 2 BTS sweaters are what I have received from this store
This shop sells mostly KPop sweaters/pullovers, shirts, and rubber bracelets. They sell mostly Exo and Big Bang things, but they also have BTS, Infinite, Girls Generation etc. In the other pictures the Exo sweater is what I received from this store. (Not bad quality but not the best either)
This shop mostly sells irrelevant Non-Kpop accessories but they do have this Exo Chanyeol panda mask. In the other pictures you can see the product I received.