Bianchi makes a belt holster that conceals well (with a jacket or shirt that covers it.
I dont remember the model and I already sold it so I cant look, but it worked nicely with a decent belt.
Check out their website for some ideas...depending on what you are looking to spend, there are a lot of great leather people that can make you a perfect fitting holster...but it comes at a price.
Also check out Safariland....I am using one of theres now and like it.
To conceal a Beretta M9 under just a poloshirt is going to be difficult.
IMHO two best holsters for your purpose are Brommeland's Max-Con V
and Milt Sparks Versa Max 2
Milt Sparks also makes Watch Six but I have no experience with this holster... never even seen one.
Just make sure to wear the shirt that is one size larger than you normally wear.