How do people get e and virtual gift cards for low prices?

Where I work, I occasionally have customers who always get so many items which include hundreds of dollars worth of lottery tickets and they pay for it all with these virtual MasterCard gift cards where it shows the barcode, card number and expiration date and a little gift box at the top and it will say redeem gift. I asked how one gets them and they just said online cheap or something similar. They are usually valid and go through but the customers sometimes forget which ones have been used up and they end up using the already used one and the machine declines it. Any information is helpful. Thank you.