I don't know any "cowboy" sports bars, but just go to any sports bar and you'll usually see Boyz fans there. Try National sports bar and grill. Great place to see a game.
Not in LA but for Cowboys games in Bakersfield, Ca (which is about 1 hour north of LA) I used to go to the Papa O's Pizza place. They show ALL the cowboys games and there is a huge following there. Be carefull though there are several Papa O's in Bakersfield and only one of them shows the boys. It is the one in the South West off Gosford and White Lane.