I am not posting this to debate whether or not buying cosplay is acceptable or valid. I think it is, and if you disagree, this probably is not the post for you. This is only to give advice for those who have decided to take this path.
Commissioning is probably the first thing that comes to mind when you think about purchasing a costume. It is a good way to get a high-quality costume, and it is pretty limitless. However, it comes with a price tag to match, and you may need to save up in order to take this route.
Thrifting is a good option, although it can require as much creativity as starting from scratch. You may look for the pieces that you need, or you may look for pieces that can be reconfigured into the costume that you want. Coolirpa is an excellent YouTube channel that demonstrates upcycling clothes. While she mostly does everyday clothes, she does a lot of costumes as well.
Buying it as pieces is very similar to thrifting. Basically, you just scour stores and websites to look for pieces that will fit with the costume. Depending on the costume, you might be able to stick to department stores, but if it is a science fiction or fantasy costume, Halloween and party stores will be more useful. This takes patience, so give yourself plenty of time before the costume needs to be done.
Buying your cosplay can be just as much fun, and require as much time and patience, as making it from scratch. What have your experiences been in buying costumes and pieces?