I've purchased naltrexone from River, United, and NaltrexoneRx. River and United both shipped the same brand in the same packaging, both from India (Nodict, as jroc states above). United is the cheapest. None of these require an Rx, but they do require you to state that you are taking the drug under a doctor's guidance. United and River both took about 4-5 weeks to arrive. The cost and bother of getting a prescription for LDN was just too much for me.
After the first month of taking LDN, I quit having vivid dreams (they were cool), so I have nothing to hold onto as to whether these really work. It's cheap enough that I keep taking LDN. I've never been convinced that LDN helps my AS at all, but I think it may help my MS though. Whichever, I don't won't to quit for a long period to better check its efficacy. I guess ignorance is bliss if it doesn't cost too much.
AS symptoms started 1991. Official dx in 2006 with HLA-B27+, fused SIJ, bone spurs in back, extreme rib/hip pain, and other family with SpA. Started Enbrel in 2006 with good results, but stopped in 2010 due to nerve damage (MS) from it. Getting good results with no-starch diet since 2011.