This is one of the most ridiculous questions I have seen on here...I truly hope this is a wind-up and that you really don't see heroin
as an option...sure you'll lose weight with it....and you will also die. Size 10-12 is not fat....you obviously have a problem with self-image.
If you are actually being serious then I recommend you seek medical help
urgently as you are a real danger to yourself at the moment. No-one on here is going to tell you where to get tablets. Please see sense and go to your Dr or speak to an adult relative. Please take care