I'm relatively new to this too so I'm not an expert but i managed to get cute paper merch (stickers and cards) from these two ETSY shops, the quality is good.
https://www.etsy.com/shop/chaerinjung
https://www.etsy.com/shop/kpoptreatsgifts
I've also bought this workout "compression" shirt from under armour because it's very similar to the style Yuzu wears for practice. It's comfortable and I like wearing it to work out, it's like a little secret and it makes me work harder
I wish I could buy more "official" merch too but it's really hard to find or very pricey when you live in Europe, unfortunately.