Obsidian can be purchased from any rock shop, especially on Lava Isle. However, you can find it anywhere on the ground on Lava Isle and pick it up for free.
If their business is in dealing with obsidian products, and they do not own any obsidian mines, then yes they will need to buy it. If however they do not need obsidian then they do not have to buy it.
Any of the Lava Isles are where you can find obsidian chunks. There is no "best" place. Also, sometimes you can find them in the rock shops if another player has sold them recently.
Wington food store =)
Type in "buy obsidian" in your search bar. You will be given information on several sources. Knappable obsidian sells for around $5 US per pound.
Obsidian shield
You can buy tham at Earton General Store :)
Wington Tack Shop
You either have to grow it on your ranch or buy it.
Treeton Pet Store for $3,000.
you can buy a guest book in the tool shop
the tool shop on tiger isle has it.
you can buy one at the tool shop