Would It Be Better If I Make My Cosplay Costume or Buy It?

Depends on your budget.  My first response would be to purchase the costume. There are many deluxe costumes that are very good.

But even cheap costumes can be altered, tailored to make you look fabulous. The best scenario is, if you or your friends have the skills, then make and tailor your costume, then buy the items that look cooler than what you can make.