Buying discount Visa/MasterCard GCs for MS purposes is usually difficult to scale/find. Two common methods are when Staples or Office Depot/Max have sales (see the dedicated thread on that; OD/OM has such a sale on Visa GCs this week). The other method is using a cashback portal at GiftCardMall - again, that has a dedicated thread.

For discounted merchant gift cards for purchases I make frequently (e.g. Starbucks, Panera) I usually search at https://www.giftcardgranny.com/ - that's not generally considered MS though.