Fineliner masking fluid
In general, Anna doesn't use masking fluid as it's not usually necessary with her style of painting.
In a small number of tutorials, Anna uses a masking fineliner to apply thin, neat lines of masking fluid through a 0.5mm pin point applicator.
The brand that Anna uses is Frisk, which is available in the UK. What makes it really useful is that it has a pin in the lid, which stops the applicator from clogging.
Below are some links to buy the type of masking fineliner Anna uses:
If you're not able to find masking fineliner where you are, try also looking for a 'fineliner applicator' similar to this one. This is essentially an empty bottle of what Anna uses and you'd just need to fill it with masking fluid.