Bulk buying is the obvious answer here, and probably via one of the High Street obliterating Internet traders. However, when you have a stockpile of things, don’t you find that you tend to use them sooner than you would have done? That was my experience with stuff like photographic film, cassette tapes, chocolate biscuits, etc, in years gone by.

Some might advocate boiling used strings to prolong their life, but where you have multiple instruments I tend to splash out on long life stings for the instruments I use most for performances, but practice at home on other instruments with strings long overdue for replacement. Someone will no doubt point out that even those costly long life strings though will deteriorate with age, even if not being played regularly.