Introducing a new piggy

Our guinea pig died last night after all attempts to get her well...
Her sister Nubbin is left alone in the cage and looks pretty sad and
is looking around for her sister. I know guinea pigs aren't solitary
creatures and Nubbin's only two, I'd hate for her to live alone for
the rest of her life, so we want to get her another buddy. We're not
sure how to do this as we've never had guinea pigs before these two.

I've heard that when you buy a new piggy, not to put them into the
same cage right away in case the new one is sick, other than that, I'm
clueless. If anyone can give any pointers, please let me know.

Thanks all,
Naomi