If you are talking about a large volume of supplies - for example if you routinely buy cases of paper for the copier, then see where the school/district has accounts. They've typically already negotiated volume purchases. Those vendors will often extend the same price to the PTO.
If you are talking about administrative/office supplies:
Check your area for a USED (or surplus) office supply store. We have a great one in our town. You have to compare prices as some things you can purchase as cheaply on sale elsewhere. But we've saved tons on reams of colored paper, nametag supplies, hanging folders, 3-ring binders, notebook inserts, bins by shopping there.
Sales (of course!). Walmart has low prices but watch the sale ads for your office supply stores, especially loss leader items to get folks in the store. If you are willing to deal with rebates, you can often get some things "free".
Ask your members to donate. Some people (like me) love to shop for bargains and don't mind donating items to the group. Some have access to free supplies. For instance, my employer deals with a lot of bids that arrive in 3-ring binders - often the huge ones that cost $10 and up. Once complete, we have binders running out our ears. Brand new - used one time. Boxes set out by the trash because there's no room to store. (If I see them I take them to school.)
See which of your members follow deal sites. If they know what you are seeking and routinely check sites like FatWallet.com or Slickdeals.com, then they can keep an eye out for what you need.
And one of the best secretes of all - GRANDPARENTS. My dad is a prime example: comfortable financially and sometimes bored in retirement. He's a deal maker/bargain hunter. He LOVES to know that we're looking for XYZ. I'm sure he (or others) would happily donate supplies. But bargain hunting is a hobby for him. He has reward points at Staples, he has lots of time on his hands and doesn't mind dealing with rebates.
My parents live about an hour away so couldn't be involved in day to day school activities. But all I had to do was tell my Dad we were low on copy paper or binders or whatever and I'd have a sack full next time I saw him.
