Your garden centre will sell it. You need to pick a line which is the correct thickness for your machine.
Ian
June 2020
You might even find there is a manual. A quick online search may be fruitful/.
did that help
June 2020
Tell us the make and model of your strimmer for better advice, but normally you'd unscrew the line spool with the plastic knob that holds it on.
I would imagine the best place to buy new line would be from a garden centre, a normal DIY store, or Amazon..
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix grass trimmers.