eBay
And on occasion the odd store with old stock. Stores in small towns and or those that have not changed hands much are also good sources.
Ebay can be a mine field. Some boxes go for pennies, others of the same go for a small fortune. Case in point, someone was bidding up vintage boxes of Gain for $$, but there were others to be had for less than $5. Gave $4 for my box of Fels Instant Detergent, but have seen the same "jumbo" box for much more.
Just keep your eyes open, you will find sooner or later.
L.