Would it not be nice if you have the skills to be able to make your own costume for your cosplay event? Some people are just handy with needles and threads and can use the sewing machine without a lot of difficulties. Making your own costumes gives you the chance to get all the details of your costume right, and it always feels good to show up at a cosplay event decked in a costume that you worked on yourself.
Unfortunately, not everyone is gifted with the ability to work with needles, cloth and thread. In this case, you can go to a dressmaker with a picture or a sketch of the costume you have in mind and ask the dressmaker to make it for you. A good dressmaker can come up with a satisfactory set that you can proudly wear to the cosplay, but you cannot expect him or her to get all the details right.
Another alternative to making your own costume is to buy bits and pieces of items that you can throw together and wear to the cosplay. This is a good option if the character you are showing up as is not a fantasy character with an elaborate costume. Japanese schoolgirls are very simple to cosplay. You just need to get a short, pleated skirt; a blazer or jacket that matches the skirt; a blouse, preferably with a wide sailor collar; a necktie or a bow; legwarmers and loafers. The downside to buying bits and pieces for a costume is that it is not always possible to be as elaborate and detailed as you would have wanted to be with your costume.