I recommend the Friends of the Suffering Souls, FOSS.
The link is here. Their website says:
Welcome to the website of the Friends of the Suffering Souls, a Catholic Lay Association conducting a perpetual novena of Masses for the Holy Souls in Purgatory.I have been a member for a number of years. You undertake to have a Mass said (in the new form or the Traditional form) at least once a year - they suggest on your birthday, as it is a date that you always remember.
This association was inspired by the 100 consecutive Masses offered for the Holy Souls by the Venerable Archdeacon Cavanagh, Parish Priest of Knock prior to the appearance there of Our Blessed Mother in 1879.
Each member undertakes to arrange at least one Mass each year for our deceased members and for all the Holy Souls.
But you can arrange as many Masses as you wish for this novena.
It is our goal to reach an average of 200 Masses for the holy souls each and every day of the year.
Purgatorial societies were always a part of the life of the Church and are still essential. There were whole Churches dedicated to it. See here, for example.
Please pray for the souls in purgatory, and have masses said for them. It is the only help that we can now give them.