Depending on your model of bike eBay would be good for a search. I have one for the 748-9** line up and it's been very useful over the years and cost about £12. It's just a laser cut piece of steel.
There was one on here not long ago which looked perfect. It was just a lump of wood cut to size. Ideal.
This is similar to the one I use for the above models, only mine is not stainless and does not have a plastic covered grip end. Ducati Chain Hub Adjusting Tool 748 916 996 998 S2R S4R Made in Stainless Steel | eBay
Old Rider has a point. What exactly do you mean. If it's a tool for releasing the hub then the eBay one looks very good. Nice n long= plenty of leverage. If you mean a tool for measuring the chain tension/movement then this is what Old Rider was referring to. It's all you need. Cut a small piece of wood 51mm long, or whatever the chain tension should be on your bike and you never need to squint at a millimetre ruler or anything else for this job ever again. Simples.
Thanks for the replies.........I actually bought one on E-Bay from a user by the name of tombonjumpi , E-Bay item number 121918559365. It`s a really well made tool and does the job easily.