Hi there, desperate reverend,
You're so right: four-string basses are pretty hard to find these days, but the industry still only makes sets of four. Image how long it's gonna take before they will make sets of 7 tuners for guitars?
You may find the stuff you're looking for at Darren Riley's site. But one tuning machine extra will cost you! My idea would be: simply buy two sets of an less famous brand for some 60 dollars (does it really have to be Grover?). They probably won't last a lifetime, but then you'll have three spares that match perfectly, won't you?