The present disclosure is directed methods for manufacturing spar caps for wind turbine rotor blades. In certain embodiments, the method includes forming an outer frame or tray of the spar cap via at least one of three-dimensional (3D) pultrusion, thermoforming, or 3D printing. As such, the outer frame has a varying cross-section that corresponds to a varying cross-section of the rotor blade along a span thereof. The method also includes arranging a plurality of structural materials (e.g. layers of pultruded plates) within the pultruded outer frame of the spar cap and infusing the structural materials and the outer frame together via a resin material so as to form the spar cap. The resulting spar cap can then be easily incorporated into conventional rotor blade manufacturing processes and/or welded or bonded to an existing rotor blade.