Not a PE teacher, but I asked this a while ago from my kid's elementary school gym teachers when I was organizing a community event. They have to go through an approved vendor and Amazon/Walmart/Dollar Stores are actually cheaper. It killed him to buy hula hoops when Dollar Tree had them for a $1.
Make sure to check Craigslist/Facebook Marketplace/Nextdoor/etc. as well. Amazon is the cheapest for things that can be shipped compactly (i.e. playground balls that ship deflated), but for things like hula hoops, the Amazon ones come in segments and Dollar Tree or Walmart are a better bet. For something like the ball pit balls, you can sometimes find someone getting rid of a trampoline with a bunch of those balls included on a local classified ad.