Where is the best place to buy a mask in Venice?
Google "Venetian masks" and you will see websites for places selling masks in Venice -- there are thousands of them. You want one made there, not from China. I don't think you will have trouble finding one!
BG is right there are thousands of shops, every second one seems to be selling masks. It really depends if you want the real thing or 'just a mask'. There are heaps of street vendors selling them or shops selling the genuine article. I wanted to buy one and spent the first couple of days of my trip just looking in shops as I was wandering the streets. That helped me see what was available and start to recognise a 'good quality' one. When I did finally settle on a shop and choose one I was given a certificate of authenticity when I bought it.
There is a shop in Dorsoduro that supplied the masks for Tom Cruise and Nicole Kidman for the movie "Eyes wide shut". It's called Ca' Macana in Calle delle Botteghe. Also Mondonovo Maschere in Rio Teraq Canal. This is apparently one of Venice Master mask makers.
Our souvenir goal for Venice was to buy masks.
I second the most important thing is to buy locally made vs. made in China (not a big difference in price) by getting out of the main tourist areas.
We wandered and looked at maybe ten to a dozen shops to find our perfect masks at the right price. If you see one you like but aren't quite sold on (like my wife) get a business card or note carefully where it is. Once you start wandering, it's not always easy to find the same place again.