For the folks suggesting prayers for the souls in Purgatory, while it is wonderful to pray for the souls in Purgatory, that is NOT praying “for the salvation of souls” as souls in Purgatory are already saved. They’re definitely going to Heaven eventually.
Praying for the salvation of souls means praying for living people here on earth, including those who are close to death but not dead yet. St. John Vianney said it was very important to pray for the salvation of the living. The living are NOT definitely saved yet and could still go to Hell.
You can say any prayer for living souls’ salvation that you want, or offer up any penance.
If you happen to get an indulgence for it, then you can give that indulgence to the souls in Purgatory while the living souls get the benefit of the prayers or other acts. I do this all the time. Prayers or communion for a living person, indulgence to the Holy Souls. Just tell God what you want and if it be his will he’ll apply your prayers the way you want.