Other than going through the airport, I haven't spent much time in Beijing but any place else in China I would say find the public market. Every public market has a "knife guy." I'm sure in a city the size of Beijing there is a huge public market some place that probably has several "knife guys." Even some of the smaller markets I spent time in Southern China usually had 2 or 3 knife guys. And that is how the local I was traveling with referred to them too, "the knife guy."