Our pto just purchased a mascot costume for our school. We used www.stumpsspirit.com
They were very easy to work with. Some costumes can be very pricey. We bought a panther costume for $399 and they shipped it the next day.
All of our schools with actual teams have mascots, I know at our high school they have changed a bit through the years and they seemed to keep them pretty updated at one time. Perhaps you can contact your schools in your area that use them and see where they got their's.
We are going to look into buying a school mascot. Do any of you have experience with this and what companies did you check with? Any I need to avoid? Thanks!!