The rustic movement of the mid-nineteenth century was characterized by designs that were made to look like they were from the country. Elegant and slim curved lines in furniture gave way to bulkier and heavier forms made from pieces that came directly from the trees often with the bark still intact.
Homes, cabins, and garden houses were designed in the rustic style eschewing classic designs. In decorative furniture this often took the form of chairs made from rough tree limbs curved to form arms and chair backs, chair legs made from tree roots growing upwards.
Stonecutters displayed a wide variety of design as can be seen in this selection of graveyard benches in the Greenhill Cemetery at Bedford, Indiana.
Note the mushrooms carved into the bench–can you spot them?
Note the heart shape carved into the bench.
Gravely Speaking 
When I was a little kid, we had a summer house on the Cuivre River that was furnished in “rustic tree furniture” …although the chairs had big cushions, I found them very strange and uncomfortable! What a great collection of “tree benches” you’ve found!
Where can I get one of these as a memorial garden bench for my grandson
I would start by asking your local marker/gravestone company. They can most likely lead you to the correct place. Good luck.