Visit your local craft store, and ask a representative if the store sells polystyrene beads for craft projects.
Ask at furniture stores, especially furniture stores that sell beanbag chairs, if you can buy replacement beads. They may not have them in the store, but should be able to order them for you.
Shop online sites like My Freaky Family and Bean Bag Refill to buy bulk bags of polystyrene beads.
Inquire at your local post office, FedEx or UPS store. They may have polystyrene beads for packaging they would be willing to sell you.
Check Craigslist for listings and visit yard sales. You may find people selling old beanbags you could recycle or bulk bags of polystyrene beads.