We’ll have to wait and see, though for FP1 the spares were there quite soon after they shipped the first batches. They did reopen the shop briefly so people could order cases with the FP1, but I don’t think spares were available before delivery of the FP1 special edition.
For people thinking about buying a battery for future use (as opposed to concurrent use as described above), remember that batteries ‘age’ even if you’re not using them:
Also, in the FP2 modules wishlist there’s already mention of a charger for battery spares. I wonder whether they’ll eventually included gadgets such as those in their shop.