I usually get deep discounts at Advance Auto. 10 quarts of PP for $3 per quart, 20 quarts of Synpower for $2.50 per quart, and once they were selling Q-Torque 5w20 for $1 per quart.
Other times, Wal Mart randomly sells 5qt containers of Q-torque for a deep discount. I also went to Wal Mart when I needed 5w40 because RTS was the only 5w40 I could find.