Orange Bitters
If you happen to own any older cocktail manuals, you will find that Orange Bitters is a common ingredient. Chances are good however, that you have never seen this available on your store shelves. Angostura Bitters is what most people reach for when "bitters" is mentioned in a recipe, but these two flavorings are worlds apart.

Luckily, during the past several years, there have been a variety of new sources available for purchasing orange bitters.

Here are some of the currently available options:

Fee Brothers
453 Portland Ave.
Rochester, NY 14605
USA
(800) 961-FEES
http://www.feebrothers.com

Regan's Orange Bitters #6
Available from Buffalo Trace
http://www.buffalotrace.com
(go to the "gift shop", then click on "mixers"

The Bitter Truth
A german company that just recently started producing a wonderful orange bitters, as well as an aromatic bitters and orange flower water.
Shipping may be a little more severe for the US audience, but I still highly recommend them.
http://www.the-bitter-truth.com

If you'd prefer trying to make your own, here is a recipe from "Charles H. Baker" for Orange Bitters:

Orange Bitters

1/2 pound dried (Seville) orange peel, chopped finely Pinch of cardamom Pinch of caraway Pinch of coriander seeds 2 cups grain alcohol 4 tablespoons burnt sugar

Mix the orange peel, herb seeds, and the alcohol Let stand in a sealed jar for 15 days, agitating every day. Pour off spirits through a cloth, and seal again. Take the strained off seeds and peel, put them in a saucepan, crushing with a wooden muddler. Cover them with boiling water, simmer 5 minutes; Put in a covered jar for 2 days, then strain this off and add to the spirits. Put in burnt sugar for color. Filter again, let stand until it settles perfectly clear, then bottle for use. Be careful not to agitate the slight precipitation or sediment during the final operation.