If it is already blessed it can’t be sold, as to do so is simony.
From the Catechism of the Catholic Church:
2121 Simony is defined as the buying or selling of spiritual things.53 To Simon the magician, who wanted to buy the spiritual power he saw at work in the apostles, St. Peter responded: "Your silver perish with you, because you thought you could obtain God’s gift with money!"54 Peter thus held to the words of Jesus: "You received without pay, give without pay."55 It is impossible to appropriate to oneself spiritual goods and behave toward them as their owner or master, for they have their source in God. One can receive them only from him, without payment.
One could give a donation, but not purchase a sacramental.
If it’s not blessed it’s not (yet) a sacramental and therefore may be purchased.
You couldn’t therefore buy holy water.
In any case, you should be able to obtain holy water from the parish.
Also from the Catechism:
1677 Sacramentals are sacred signs instituted by the Church. They prepare men to receive the fruit of the sacraments and sanctify different circumstances of life.
1678 Among the sacramentals blessings occupy an important place. They include both praise of God for his works and gifts, and the Church’s intercession for men that they may be able to use God’s gifts according to the spirit of the Gospel.
God bless you
