My wife and I are looking for a bi gal to play with both of us in TJ. I know I can go to an escort service but that's just not as much fun as find THAT girl in the bars.

Is it difficult to find girls that are bi in the bars? Will my wife be well received going into the Hong Kongs of the world?

Lots of bar girls are bisexual.  However, many are not open about it.  

Finding a bisexual street girl would be harder than finding a bisexual bar girl.  And probably less fun.

Although you don't want to contact an escort agency it would be much easier and quicker to obtain a bi girl by contacting an escort agency than finding one bar hopping.  I suggest you carry the phone number of some agencies with you in case you strike out in the bars.  (Mexico Lindo Bar has a reputation for putting together dates in short order, but don't wait until the wee hours to contact them or any other agency.)