The impulse to obtain a Mass card is one of those things that arrives out of the blue for many people, usually when the death of someone close spurs a desire to help them rest in peace through the Sacrifice of the Mass.
When priests celebrate Mass they usually include an intention for people in need. The faithful request that a priest make their intention his own by a Mass card, with a cash offering as a small personal sacrifice.
If parishes sell Mass cards then acquiring them is easy. If not, people can pick them up online from Catholic outlets, and even on Amazon. Mass-produced Mass cards are not to everyone’s taste, and if you don’t like them you can make one at home, perhaps with an image of the person for whom Mass is intended. Make sure to include your contact details, the name of the beneficiary, the preferable date for the Mass, as well as an affordable donation.
Such offerings are for the upkeep of priests, and if the priest cannot offer Mass on the requested day, he might sign the card and pass it on, with the gift, to another priest or Religious, often those working in the missions, with the agreement of the donor.
You may wish to arrange for a Catholic charity to benefit from your Mass stipend. Many of them arrange for Masses to be offered.