The traditional shaped nagiri gami are available through the Shureido UJSA website:
These jars are used in pairs for developing the gripping strength and the bushican strike. They also develop the trapezius muscles, forcing the shoulders forward and down.
The jars on the http://www.bushifitness.com/
website aren't available anymore. I have a set and they are excellent. They are made of a resin material that's injected molded and they won't break if accidently dropped.
The jar used by Toyama Sensei in the photo above posted by Sensei Canna is a completely different jar. It's shourter and rounder than the nagiri gami on the Shureido web-site.
This jar is used by itself and in gripped with the palms rotated inward as shown in the photo but with the arms in Sanchin position. It develops the deltoids and forearms frp, a differemt amgle.
One might use a medicine ball for a similar effect. Or, use a bucket or pail or large round crockery planting pot.