one

It’s better to bring a phrasebook, but if you haven’t got any, don’t worry.
It’s better to bring a phrasebook, but if you haven’t got one, don’t worry.

Use one to avoid repeating a singular countable noun: ‘Do you need a pen?’ ‘No, I’ve already got one.’

We can use any in negative sentences to avoid repeating a plural or uncountable noun: ‘I couldn’t find any oranges.’ ‘There isn’t any milk.’