heh, I thought I was he only one who looked into this. I went to lowes to look for gypsum. Most products had chemical additives or were sold in large quantities. However I did find a product called Gold Bond Accelerator. It is used to quicken the set time of plaster. The ingredients listed are gypsum and quartz and it is sold in a 24 oz tub. The quartz content is <5% according to the MSDS.
The material safety data sheet says this about ingestion: "Ingestion: Gypsum is non-hazardous and no harmful effects are expected upon ingestion of small amounts. Larger amounts may cause abdominal discomfort or possible obstruction of the digestive tract. Seek medical attention if problems persist." The main risk is inhalation of quartz dust.
Two things to note:
When I added the gypsum to water most of it seemed to dissolve, but some of it would not dissolve, and settled to the bottom of the bucket.
This gypsum is light grey/tan, but some of the gypsum I have seen in LHBS is closer to light grey/white. Dont know if that means anything.
Another salt that is easy to get is epsom salt (MgSO4). You can get in a carton from a drug store. I got some from CVS that is ok for digestive use (as a laxative) in small quantities.