My answer.....all three!! There's nothing wrong with buying your cosplays pre-made. There are beautiful options out there, and this is often the most affordable in terms of time & money! Another option if you don't know how to sew is to commission someone more talented than you to make exactly what you envision. Lastly, if you're talented - you can sew your own!
In most cases, I do a mix of two or all three of these ways. I might make my accessories and buy the costume, or vice versa! As I've been busy, I usually just buy or commission everything these days :)
The costume pictured above (Bad Girl from No More Heroes) was my first successful, handmade cosplay! Painting the fabric and lining up the stripes was not easy, but I was so happy with the end result!