If you can read some Chinese there's a very good website with a listing of TCMA teachers in various cities (including Tianjin) at:
http://www.niudor.com.
Agree with Jonathan, Yang Hai is probably your best bet as he is (a) actually from Tianjin and (b) is a (very good) practitioner of Cheng style bagua himself.
If you're looking for other suggestions, AFAIK there's also:
(a) Ge Guoliang (戈国良) teaches Gao style at the Nankai university campus at the weekends
(b) Tang Shengjun (唐胜军) teaches Cheng style bagua (not sure which branch) - his great grandfather was a famous xingyi master (Tang Weilu) tel:15102245753
I've also heard that there are Yin style people (Cao Zhongsheng branch) in Tianjin, could look for more details if you're interested.