Just in case, I searched for something to use as a weapon.
Literal
By-chance [for-に備えて], weapon [into-に] become thing [object-を] searched.
武器になるもの ('something that can become a weapon') is a relative clause modifying もの — '(the kind of) thing that turns into a weapon.' になる here expresses potential or suitability. A typical survival-scene sentence.