You can order single remedies from any of the following homeopathic pharmacies. Hahnemann Labs (in California) and Helios (in the UK) have many (or most) of the newer or unusual remedies.
Hahnemann Labs: 888 427 6422
Helios Homeopathic Pharmacy (UK)
Homeopathy Overnight: 800 ARNICA 30
Natural Health Supply: 888 689 1608
Remedy Source: 301-276-7709
Washington Homeopathic Products: 800 336 1695