Even at 15 they are limited to how many hours they can employ you for per day/week.
And also, minimum wage doesnt come into effect until you reach 16 years of age so your unlikely to get paid much more than you are as a paper kid.
Garden Centres are a good source for work for teenagers as they take them on to water the plants at weekends and during school holidays to save the older staff doing it and letting them get on with other tasks.
Have you considered working for the newsagent you do the paperround with? They often employ people to sort the newspapers for all the delivery kids as well as delivering them. Maybe they could offer you work doing that or even shelf filling for an hour a day.
Whatever you do, your age is very limiting to your options as most employers wont take on under 16's due to laws and also maturity of the person, and those that do wont pay very much as you are neither skilled, experienced or much use to them than a general dogsbody that does some gopher work (gopher = go for this, go for that).
Keep looking around tho, dont let the first door shutting stop you getting a job if your dedicated to earning some money, as it is a worthwhile thing to want to earn your living instead of scrounge it.