Every KPop fan knows this problem. There are plenty of useful sites that sells KPop albums but there is a problem when you try to get some other things from your favourite artist. What to do then? Don't worry. There are many simple ways to get what you want.
First of all you can try to find official online stores of your favourite KPop group or soloist or even the whole entertainment company. For example thisYG Entertainment shop or Bangtan Boys official store. You can always try to use google search engine to find what you're looking for. You can type something like 'EXO official store' or 'Got7 online store'.
The second thing you can try is buying from sites like Ebay or Amazon. It's really easy and convenient way. But it can be sometimes difficult to find something interesting (there are literally millions of products!). You can always try to find some inspirations on many KPop related websites like KPop Merchandise World. You'll certainly find there some good stuff like EXO computer mouse or Big Bang jacket with a link to product's page on Ebay or different websites. It's making your shopping way more easier. If you are an EXO fan you can find many interesting items in this EXO merchandise category for example.
The most interesting items that the fans like are:
fashion items like T-shirts or jackets
trading cards that are highly collectible
albums with photobooks
light sticks (very helpful at concerts)
You can easily explore the site checking many categories like Got7, Big Bang, Vixx and BTS merchandise. A great example of the items you can find there are this EXO sweater from EXO merch category.
There are also sites like Pinterest or Tumblr where you can find good examples of what you can get on Ebay or Amazon but probably you'll have to find these products yourself since there won't be any link to guide you.