Cascade at Walgreens
I also saw boxes of phosphated-Cascade at Walgreens. You buy a box on sale for $3.99 and they give you a $1.00 discount coupon at the cash register to use for your next purchase of whatever. It's a good price for Cascade.
Stocking up on bulk dishwasher detergent powder makes me wary. I think the salts absorb moisture from the air and over time cause the detergent to clump together. One box I recently purchased says to "use within a few months after purchase."
You can manually break the clumps, but it complicates or prevents pouring straight from the box.
I prefer using bulk powder, so I can adjust the amount of detergent, depending on what I'm washing. For multi-year storage, though, the "Action Paks" might be more convenient.