How to get market size data for calculating TAM?

Take a look at competitors and related companies. Have they disclosed any data? Or any articles written about them, with facts researched by journalists? You can also do a top-down analysis trying to find the data from the likes of Gartner or Forrester (and try not to pay an arm and leg). Or you can also do a bottoms-up approach, how big is your addressable market, how much are they willing to pay etc. If you are truly creating a category from scratch (many people think they are, but in reality it's rare), then you need to find 'traditionally' similar markets, and extrapolate/best guess your data on comparisons. At the end of the day there's a mutual understanding that TAM data is not an exact science.