Is there any in game voice chat system? In what app it's easier to find players that use voice communication? Teamspeak? What server?
There is built-in voice chat support in Left4Dead 2.
For PC, the default key is C.
For XBox-360, just plug your mic in or turn on the wireless mic.
Blueraja nailed it (+rep).
Also, as of one of the later patches, voting to turn on "alltalk" has been introduced in game. Which is, everyone can hear everyone else, no matter what team. Press escape in the game and go the vote menu to start an alltalk vote.