Unless you buy organically grown grains you can assume that some pesticides & herbicides were used in it's production no matter if you buy it from your local feed store or purchase brand name brewing grains. Depending on where you live barley may be hard to find at the local store. There are no suppliers in my part of North Carolina that carry barley feed or seed. It is sometimes available as feed but because of it's cost compared to other grains it's pretty scarce around me.
Take a look through any beer suppliers list of malted grains and you will quickly learn that there is a lot more to it than just barley. There are easily 50 or more types of malted barley used in brewing. I looked at growing and malting my own but quickly realized that that was an entire hobby/job in it's own right so I decided to focus on brewing beer and growing my own hops.
As for using wall plaster I would say no. Yes it would adjust the PH but you don't know all the other chemicals (plasticizers, bonding agents, anti caking agents...) added to make modern plaster or drywall mud. Any chemicals you want can be inexpensivley purchased from a home brew supplier.