Both are good options. If you make your costume you know it’ll fit you just right, if you buy your costume and done and you don’t have to spend hours putting it together. I usually go somewhere in the middle. I buy pieces for the costume and then modify them to my needs. Now. your time constraints, time and money options make a huge difference for what you do. For something like my dad’s green arrow costume I got him a pair of cheep black leggings from the junior’s department of a department store and then the right color green tee-shirt from a craft store. I then modified the shirt to look more like Green Arrow’s, took the old sleeve’s from the shirt to make the gauntlets. I made the hood out of old green fabric I had lying around. It’s about being smart with your time, I find. Sure, I can make a tee-shirt but when I can buy one already made for less than the materials cost me, I’m going to buy that shirt. For Kid Flash the leggings were bought as it. The shirt was just a bright yellow running shirt so we added the symbol. Then I found this great tutorial on Youtube that helped me figure out how to make the mask. Most of my costumes come from scratch because I can’t find a non-cheesy option for the characters. Sure I could buy the Dalek dress from Hot Topic or any number of places but they all looked cheap and I have this desire to look like I do know what I’m doing when all of my creations.
It all comes down to money vs. time. I just started my Star Trek costume. I made my coset out of Duct tape and a tee-shirt. The whole project cost me 15 bucks. I can’t afford a real corset, I wish I could but I can’t and I have the time to make one. However my shoulder piece was a sweatshirt I found on the clearance rack and got for less than 3 bucks then cut it up. There are so many great options and so many places in between making and buying, so why not use the best of both?