If anyone has a Sprouts (similar to a Whole Foods) store, check their canned veggie section, I did find pumpkin there (Farmer's Market brand, organic) for about $2.50 a can. (Gee, maybe I shoud sell it on eBay and make a profit!   I do keep it on hand for when one of the dogs has tummy trouble....mixed with baby rice cereal and a little canned dog food, it can be beneficial for both constipation OR diarrhea, and most dogs like the taste.
Another thing you can try is a little mineral oil...I had a dumb Siberian Husky that ate a bunch of sand once (managed to dig up one of the patio blocks in her kennel) and it blocked her up good. Vet recommended mineral oil, it did the job...she pooped oily sand for 2 days...
I do tend to agree with the thought of dehydration. One thing you can do to get more fluid into dogs in the hot weather is freeze some low sodium (excess salt is not good for them) beef or chicken broth in ice cube trays and give them the tasty ice cubes for a treat. Mine love 'em!