Alice, and anyone else wanting to know about etiquette . Many Catholic churches, whether Roman or Eastern rite, will have two or more holy water fonts as you enter the church. Past the vestibule. Into the Nave (pew area.)
Bring a little bottle. Take an ounce. If you see the poor box put in $1, or nothing.
Sometimes there may be a Baptismal Font instead. There is holy water in there too.
If the doors are closed, you can go to the rectory. That is the house next door. Use the office door, and you can usually find the churchlady there during business hours. Someone is always around and can help you out. Please don't use the Priests private entrance to his home.
If, for whatever reason you need to dispose of holy water, pour it into a garden, or any earth outside.