When I was traveling overseas I bought a number of Propper shirts. They were a knock off of the 511 gear that fit me better. Since 511 dropped that style they are about the only game in town for hidden pocket shirts.
This type of shirt.
http://tacticalgear.com/propper-lightweight-short-sleeve-tactical-shirt-coyote-tanWhat I liked was the zippered hidden large pockets in the front. They held plane tickets, itinerary, passport, extra cash, etc. all secure and where pickpockets would have a hard time getting to it. When I got back home, I found they work great for holding a small pistol. I have carried my R9 that way.
The zippered front make for easy access if you want to wear a shoulder holster. (The front buttons are fake, there is a long zipper behind all but one of them) I tried a Kangaroo holster, but did not like it, but the shirt did it's part.
Some will complain about safety as they always do with shoulder holsters, and would probably have a fit about a lose handgun in your pocket. But it has been a choice I have made in the past. With the zipper being on the side, and the pocket behind the normal shirt pocket you can position the handgun so that it is easy to open up and get a good grip before exposing it. You could put it in a small holster if you like, but that might add too much bulk.