Marathon offers both sided and sideless pre-wound bobbins.
They are available in polyester filament and spun polyester in both white and black.
Plastic sided pre-wound bobbins are also available in colors!
Polyester filament thread is the standard type used for bobbins.

However, spun polyester is more economical and offers that soft look.
Keep in mind that spun polyester thread tends to leave lint residue in your bobbin case after a long use.

Spun polyester is also available in large cones, which can be wound down.
This is the most economical choice when cost is the main factor in choosing your bobbin.