Like all costumes, there are places that specialize in anime costumes. The previous answer gives two places you could go; I’ve never looked at these stores, but I assume the previous answerer knows more about anime related costumes than I do.
However, I will echo their point about YouTube. YouTube has so many good tutorials for how to make specific things. They may also have clips from the anime itself that you could study if you wanted to make your own costume.
Do be warned that this can be expensive, especially if you do not have access to a sewing machine the first time around. Still, depending on what costume you’re making, you may have a lot of costs due to materials and the amount of each material.
Yes you can, and most people do. They just look online at one of the many stores who specialise in Anime wear and select an outfit that resembles the character or character type that they wat to portray.
Pick a store with a good range. Hokagestore is one of these and Soguishirts is another. You could also look on youtube if you fancy working on an unique anime design yourself.