I'd listen to Rob's advice. If possible, practice on the graft you really intend to do. It makes no sens to practice on grafting avocado which is somewhat easy when you really intent to graft mango (hard to graft). it is even silly to practice grafting mango when really you intent to graft lychee.
Grafting is not just the cutting. It englobes the understanding of the nature of the rootstock and the scion.
But if you're objective here is to find a cutter, then a utility knife like a box cutter, retractable at desired length, would do just fine. I've tried the most expensive grafting knives endorsed by so called "pros", believe me, they're all snake oil.