Your best bet is to join a gym, because the bigger gyms always have several yoga classes per week which are included in the membership. Haoheng is one of the better value gyms and they have several branches. Around 1600 for a year. Not sure what there is in Beichen but am sure there's at least one decent gym. I only know of one specialised yoga studio called Yutao yoga in Wangfujing but it's very expensive and of course doesn't include a gym.