Hmmm I had a job when I was 15 so it was probably a little easier for me, but there are some things you can do…
1. Put all of your change in a jar. This is a bit easier for Canadians because our change comes in values such as $1 and $2, but even a bunch of quarters will help. It’s surprising how much can add up.
2. See if you can do something like babysitting or tutoring - you’re the right age for parents to hire you for younger kids (13-year-olds usually have trouble babysitting toddlers, for example).
3. Mow a lawn/do chores for elderly neighbours, etc.4. Consider doing art commissions for friends - or if you do crafts try selling those.
5. Ask your parents to help you shop online. You can get crazy deals if you’re based in the US - often with free shipping.
6. Plan cosplay around holidays and ask for items/money as gifts.
7. Have a garage sale to get rid of some old stuff for cash.
Also - try making cosplay mockups out of cheap fabric if you’re going to sew from scratch so you don’t waste nice fabric on silly mistakes.
I used bridal satin; I used the right side for the white fabric and the wrong side for the blue.