Yes, you can get it at Walmart. I think it's usually in the baking aisle.
 
Here's the link here:  
http://www.yorkietalk.com/forums/showthread.php?t=98926 
It does say for constipation, but I've read that it's for both.....I hope you can get him to a vet though. How scary. BTW...if you don't have time to read the list, it says 1 tsp for an adult.