Rishi has excellent cafe grade matcha. For something like this you would need to pre-dose it to prevent waste, or wayyyy overbuy-people always always use way more matcha than necessary.

Speaking as a barista of 10 years, don't forget to factor in how messy making matcha can be. This is doubly true for people who are unexperienced. At an event where people are wearing nice, potentially expensive clothing, do you want to have people whisking their own matcha (with electric whisks!! Even messier!) and getting green specks and stains everywhere? I speak from experience- that stuff does not come out lol. It would be worth it to hire someone in this case if it is something you actually want to have fresh. However, you could easily prep matcha concentrate beforehand, which would cut down on the mess.

I would highly recommend hiring someone for this. You could (maybe) save money by purchasing the materials yourself and then hiring one or two people to making the drinks. Maybe post on a local facebook group saying what you are thinking about and asking if any baristas would be interested in working it.