For those looking for Prell Shampoo, look at the Dollar General Store or similar.
An employee at Walgreen's suggested to check with the Dollar General Stores. I called the local store and they claim they have Prell.
The employee also suggested I call the phone number on the Prell bottle. I did and found out the number has been disconnected/no longer in service.
Since the customer service number on the Prell bottle has been disconnected, this probably means Prell has been discontinued and outlet stores are selling the remaining bottles.