How to indicate that a passage is to be played on only one string?

Hello.
I have been composing a string quartet and would like to have the second violin play a certain passage on only the G string to give it a warmer tone. As many of you know, this is usually represented by a Roman numeral above the first note, and sometimes as a line, extending over the passage, each numeral representing a different string (e.g. I=E II=A, etc.) Can somebody tell me where to find these numerals for note input?
Thanks in advance!