You'd come out a lot cheaper advertising for them on eBay, CraigsList and elsewhere and buying them directly from consumers. I've had this experience with computers, stereos and large appliances -- picking up used items on a cash-paid, as-is basis directly from consumers is a cheap and abundant source that usually beats companies selling refurbs or castoffs, since you're getting them from the same place as they are but not adding in a layer of profit.
You can also find a few by printing up some business cards and going to cellular stores around your area. Some refurbish them, others sell them on eBay, but mainly, for your average local store, if they have a guy that will just come pick up traded in phones by the box for cash they will be amenable to a deal.
You can also generally fill your car with them at local pawn shops and flea markets, again far more cheaply than buying them online (no markup, no shipping, etc.)