Adhesive for a Decorative Cast Stone Facing?

My home has flat stones on the front cemented on. They are decorative and look like flagstones, but I think they are actually castings. One or two have fallen off. They are approx one foot on each side. I prefer not to have to mix up cement or mortar, if I can avoid that. Can you recommend a good ready-to-use adhesive that I could find at Lowe's or Home Depot to glue these back on?