I'd try Home Depot or Lowe's Gardening Centers. Online, you could try PriceGrabber.com, one of my fave sites. Hope this helps.
Check the sporting goods department in Wal-Mart or K-Mart or.... These areas carry packs of pegs where the tents are found since the plastic ones can break.
If you just bought them and don't want to go back to the store to exchange them, look for a toll-free number printed on the box or on instructions. Call and tell them your box was missing some stakes and they should send them to you free of charge.