I just get them all from online leaderboards.
Bastion, Ms Splosion Man and Trenched all have leaderboards for the story mode, so you can see everyone who buys it. I got HMH's number by seeing the completions of the first level.
Gamasutra does an analysis every month as well, though usually it's well into the next month, so it is generally kind of outdated.
http://gamasutra.com/view/news/35958/InDepth_Xbox_Live_Arcade_Sales_Analysis_July_2011.php
--
omg did u red text from last night dats so me LMAO : )
im not a slut i jus liek 2 haev fun LOL : )