Hey there! I would search for a hand calligrapher in your area and my guess is that you would provide them the mirror and they would write the seating chart in it? Normally a printer would provide a material to print on, but I’m thinking the mirror may be something you’ll need to choose and purchase.
I looked into a premade mirror welcome sign and they were ridiculously expensive. What I did instead was purchase a mirror second hand on Facebook market place and ordered a decal off Etsy. They are super easy to apply and look great.