Coop stores sometimes have a permit to employ staff under 16, it depends on your area. But ask the council. They will know for sure. They will be glad you asked as it means you're not going to get involved with the wrong crowd, and it shows initiative. They should support you to find something. If any adult cannot help you, try someone else - explain how important it is for you.
Statistics on employment for under 16s show a massive decrease over the last 20 years in employment, but from memory I think it's one in ten 14-15 year olds have jobs now, so there is hope.