Well there are many places in delhi. But the most known are janpath, sarojini and lajpat Nagar.. all these markets sell almost same clothes.. however if you're more Keen to buy a branded jacket at cheapest rate then opt for Himachal bhawan in Mandi house .. they always have some kind of huge sale.
graffiti lane is sarojini nagar has some great pieces but u really need to give them a good check before buying!
1- sarojni Nagar market
2- janpath market
3- Uttam nagar- Arya samaj road
4- Karol Bagh market
5- lajpat Nagar
All of these places will provide you trendy yet cheap jackets