Can I buy more disc space?

Hello,

I have gone through every option of optimising storage. I cannot delete any more apps/photos/documents etc than I already have but am left with 8.28GB available of 121.12GB. I have also optimised storage, stored in Icloud, emptied trash automatically and reviewed files to reduce clutter. I am being reminded constantly to update my Macbook Air but do not have enough space to do so - I'm now pretty fed up and ready to switch from Apple completely if there is no option of buying more space. Am I left with now having to buy a new Macbook with more storage?