Kombu is a kind of wide, large seaweed. In Japan, it is usually dried and sold as is (see picture below). However, I think just recently, Japanese have started transforming it into powder form. I say recently because another mom here first heard of kombu powder from me (in 2017) and even my husband's parents (and his father is a professional cook) doesn't even know about it until I brought one to their house (in 2013).
I find it quite tedious to get dashi from kombu itself (because it usually takes 30 minutes to 10 hours, depending on if you're boiling it or just soaking in water to extract the flavor) and maybe Japanese people have started thinking so too, that I've started seeing kombu powder sold in common supermarkets.
However, the first kombu powder I used was the one I bought at Taichu Vegetarian Grocery Store in Kyoto (which is around 15 minutes on foot from Kyoto Station). They call it Kyō-fū kombu dashi (meaning Kyoto-style kombu seasoning) and sells it for ¥810 (500g pack) and ¥1190 (1kg pack). Of course I always buy the 1 kg pack (topmost picture). If you want to buy their Kyoto-style kombu powder or browse over other vegetarian products they sell at that store, please read the blog I wrote about them.
If you prefer to buy kombu powder at the supermarket nearest to you or in Japanese or Asian grocery stores, perhaps it will help if you have a picture of what you are looking for, so here is a picture of a pack of kombu powder I saw at the supermarket near me (see pictures below).
If you can't speak Japanese and the store clerk cannot speak your language either, just show them those pictures and I'm sure they'll help you locate that product or a similar one if they have it. That's what I used to do during my first months here. After all, a picture paints a thousand words.
If you already have kombu powder and is thinking how else you can use it, try my recipes! Though I'm sure you can put kombu powder in any dish, because that's what I do: mushroom powder and kombu combo.