Fee Bros. and Regan's Orange Bitters
Go to Wine & Liquor Depot in Van Nuys -- it's on Saticoy just west of Balboa.
Wine and Liquor Depot
16938 Saticoy St
Van Nuys, CA 91406
I bought Fee Bros. bitters a couple weeks ago. Not orange, but I remember having to decide which one to get, so I bet you could find it there.
In any case, it's a great huge liquor store. Good selection of single malt scotches, too.
Barkeeper on Sunset Junction has a tremendous selection of Fee Bros. Bitters, orange included. Not the valley, I know, but depending on what part of the Valley you're talking about it could be close. If not, it is worth the drive anyway, especially if you need paraphernalia to go with the bitters.