The best way to find a job would be ideally to look, and ask in-store.
BUT this isn't the only the you can do, personally I'd say the best thing to do, is to go onto we'll known retailers, and just work your way to the bottom of their web page, and look for 'careers', and simply look to see if there's any vacancies that way.
what Indonesia was looked at my local shopping centres website, ( Westfield, Stratford ) and went to a full list of their stores, and from there went to their websites and applied via there. Looking for a job at that age is very hard, but you are more likely to be employed as the company can get away with paying you less as you'd be under eighteen, watch out for that and don't let them take liberties with you.
Best places to look would be;
JD Wetherspoons (Let you choose your hours)
Schuh (Good atmosphere and really good pay)
Any supermarket
River island
Republic
Vue cinemas
Good luck job hunting and let me know how it goes!