It's finding the vacancy as opposed to getting the job that is difficult. Supermarkets have labour budgets which are relative to the sales - so in this climate where sales are generally down, supermarkets aren't employing new people, even if staff leave they often cannot be replaced. It's a ridiculous scheme but it's just how it works (in Sainsbury's anyway)
Working in a supermarket isn't all that bad, you just have to have a life outside of it, and make friends with your collegues quickly. Without that, you won't enjoy it at all, and despise every moment.
Where I work a lot of staff employed recently had family who work for the store which gave them an unfair advantage over other applicants.
Alternatively have you considered fast-food joints like Burger King or McDonald's? I worked in a McD's and they paid well and had great benefits - ok the work wasn't great but the moeny overrides that in my opinion. If you can stand the stigma of working at somewhere like that, they are all employing at the moment, having done well financially in the recession.