What can I add. For new stuff I doubt there is a better place than the IDEA World Fitness Convention. Depending on where you live, you may be able to find some great deals at used sports equipment stores. . Power Systems and Perform Better are excellent resources, and you can also find some great deals on e-bay if you have the time and patience to search.
I have never worked for a gym and have only been in my own studio, so I have had to search for the best prices for everything since I was starting from scratch. I happened to find a gym that was going out of business through CraigsList and managed to snag a bunch of cheap weights off of the owners hands.
Big Fitness also has some great prices on things.
However, Amazon is my go to place, since I can easily shop around and find the best price while also seeing all the customer reviews, which help immensely.