If you have a PC and are looking for gift cards to the Microsoft Store, one of the best ways is to use Microsoft Rewards.

Microsoft rewards redeem giftcards page
You can earn rewards each time you perform a web search through Bing on your computer or smartphone and earn bonus points by taking a daily quiz and other quick and fun activities that test your brain and help break up the daily monotony you might experience at the office or home.

Gift card options are very limited. You can redeem your Microsoft Rewards points for gift cards to the Microsoft Store for the next time you need to make a purchase like your Office 365 subscription, app, movies, and games.