There are two different types of Argan Oil, you will need the cosmetic one (the other type is for consuming/eating and tastes really nice!).
Just massage it into your skin and it will help with arthritis pain, I use it occasionally on my knees in the night and by the morning the pain is much better. My father also uses it on his shoulders and says it eases his pain considerably.
Expect to pay around £13 for 250ml bottle. Make sure you buy from a reputable place (otherwise you may end up with some other type of vegatable oil). I always stock up from Herboristerie Firdaous (near the Hotel Tazi, if you need directions then feel free to email me). This shop has been around for years and their prices are fixed (no need to haggle).
You can also buy Argan Oil at co-operative factories on the way to Essaouria, I have visted quite a few of them and it is very interesting to see how they make the oil, the prices have always been way higher than they should be there (even though they are mean't to be the cheapest at these types of outlets).
Get as much as you can, if you have to buy in the UK it will cost around 4 or 5 times as much as you will pay in Morocco (I once paid £20 for 50ml cosmetic argan oil in the UK).
Edited: 8:35 am, May 18, 2011