Here are the ones I've got, and where I got 'em.
Angostura: Pretty much anywhere. Ango Orange is also widely available at most Binny's.
Fee's Orange, Peach, and Old-Fashioned, as well as Peychaud's: Sam's on Marcey (Lincoln Park)
Regan's Orange #6: a liquor store in Washington, D.C. Haven't seen it here.
When Kyle McHugh had just opened DoD, I asked him if he had any bitters -- he didn't, and gave me his personal bottle of Fee's Grapefruit, and I haven't seen them for sale since I've been last. Maybe I've just missed them?
Bittermens' of Boston is going to start distribution of their Grapefruit and Mole bitters, and I couldn't be more excited.