If you work in the entertainment business you will likely have a constant need for costumes. Finding somebody who can supply them for you can be very simple or very difficult depending on what you are looking for. The best option for finding a costume supplier in most cases is to find other people who work in the entertainment industry and ask them about their costume supplier. This should make it fairly easy to find somebody good.
the Yellow Pages and look up costume suppliers. There are actually quite a few of these. However most of them are set up to sell costumes to individuals for things like Halloween. That doesn't mean that they won't be able to help you, they probably can, but you may not get the kind of original costumes that you need. If you needs are for fairly common types of costumes you will probably do just fine with one of these companies.
The other option that you will likely use to find a costume supplier is to go online and look up costume suppliers in your area. This will likely provide you with similar results to what you get from the Yellow Pages. However the advantage of doing this online is that they will likely have pictures of most of their costumes available online. This will allow you to assess whether or not they can meet your needs without your having to actually go there and check them out yourself. However you will likely want to visit them in person before you start ordering costumes from them.
Where you are really going to start to have problems is if you need to buy costumes that are unusual or different from what the normal costume shop would carry. In that case you are likely going to need to have them custom made. There are lots of tailors and seamstresses who can handle this for you if you know what it is you are looking for. If not you are going to have to hire a designer to come up with something for you. This can be pretty expensive but if you need something unique it is really your only option.
There are several ways to find somebody to design costumes for you. The internet and Yellow Pages are certainly options. However you will likely do better if you get a personal recommendation. You almost certainly know other people who work in the entertainment industry. You should definitely ask them if they have any experience with having costumes custom made. If they have you should ask them about their designer. Word of mouth is an important part of doing business so find out from others in the industry about their experiences with their designers. This will help ensure that you get somebody good who can supply you with good quality, unique costumes.