Now, I've never seen any Nanoha, but my understanding is that there's at least some environmental mana. If the crossover fics haven't lied to me, drawing on ambient mana is part of how the Starlight Breaker can be so huge. Another thing I've picked up is the existence of "Cartridges", consumable mana sources (like magic alkali batteries). If you can't cast anything yourself, you can build the tools. Or build the tools to build the tools, if necessary.
EDIT: Because I think in terms of Magic: the Gathering, I'll phrase it thus: just because you can't cast spells doesn't mean that you can't activate abilities, even if you do have to build the artifacts with the abilities the hard way (and beg, borrow, or steal as many Moxen and Lotuses as you can manage, since the M:tG equivalent of "unable to generate mana" would be something like "unable to form mana bonds with lands").
EDIT: Because I think in terms of Magic: the Gathering, I'll phrase it thus: just because you can't cast spells doesn't mean that you can't activate abilities, even if you do have to build the artifacts with the abilities the hard way (and beg, borrow, or steal as many Moxen and Lotuses as you can manage, since the M:tG equivalent of "unable to generate mana" would be something like "unable to form mana bonds with lands").