Thanks cowboy123 for reading our post and commenting. You're right in pointing out what this post is not: A comprehensive guide to all the different kinds of mooncake one might look for in the city. What we did do was list all the places that we knew mooncake was readily available. I did a quick search and Wing Wah is not distributed in Beijing as widely as it is in Hong Kong, but I did come across this blog: http://blog.sina.com.cn/s/blog_48ffaa830100takf.html.
Also, as for Beijing-style mooncakes, some of the gift boxes listed above may include Beijing-style (as well as Suzhou-style, Yunnan ham, etc.), but we felt it would get tiresome to list all the options for each. If you're looking specifically for the Beijing type, I would suggest the old standby for Beijing snacks, Dao Xiang Cun.