For some websites like MYNTRA and JABONG you dont need any coupons as for every product on these websites, you will find a "CLICK HERE TO VIEW OFFER" link, clicking on which will give provide all the applicable coupons on the day.
For others, a good way is to get a mobile recharge from FREECHARGE or PAYTM. On FREECHARGE, coupons depend on the amount of recharge, however there is no such restriction with PAYTM. Here you can get coupons with a recharge as low as Rs. 10/-
Regarding third-party websites that provides coupons like couponsduniya or couponrani, most of the times these websites coupons that are either expired or are just the link to websites offer page. So, personally I don't rely much on such websites. Though you can get some coupons there but chances are rare.
Other source of coupons is your credit card. If you own a credit card, you can check deals on the webpage of the credit card company. For example, with SBI credit card, you can check the deals under HOT DEALS tab on the SBICARD webpage.
HAPPY AND DISCOUNTED SHOPPING!!