6-8 safe days isnt very much but I would rather be extra safe ! I can usually tell when I ovulate so I usually go by that. Mymonthlycycles.com does tell me when I am fertile and I just extend the days. It does tell you not to use the site to prevent pregnancy but so far it has been on the money with my fertile times. You could always buy a fertility monitor to track your days also.
I have seen lambskin at the pharmacy but they were very expensive thats why I went with the polyurethane.