Craigslist and Ebay are great resources for used equipment. I have had some success in finding some good deals over the time. You need to be patient but they will come. I have found that ordering through online can be very expensive (Due to shipping costing nearly as much as the product) so Craigslist and eBay (input zipcode) you can navigate some local deals!
Fuel the Movement,
I have used mostly discount stores like Marshall’s, TJMaxx, Ross, and Play it Again Sports. Dumbbells I usually buy new so they aren’t rusty or torn up but Play It Again Sports is cheaper than other Sporting Good’s Stores. I paid full prices for a step and risers for safety reasons. Of the few years I have been in business, my clients have given me equipment that they purchased but never used and any time I see something, like a medicine ball, I grab it up. My massage therapist used to be a PT and she gave me stuff. Networking and letting folks know what you need and it appears.