http://www.undiscoveredscotland.co.uk/areawestnorth/self.html

The above should give you most information that you need, however as has already been said most people on the islands are generally OK about camping on their land as long as you look after the place.

But please remember that the machair ( http://en.wikipedia.org/wiki/Machair ) is all ecologically precious and you should never camp on it and also take care not to disturb when out walking etc'

Wrap up warm ... even in the summer it can be a wee bit chilly!

;-)