Cords and Slings are a very versatile piece of gear for a climber; given the myriad ways they can be used in.
120 cm slings are the most versatile and work both as Personal Anchoring System and for setting up anchors for belay or top rope. The two materials have their own pros and cons and these have been discussed at great detail. Locally, Decathlon and Petzl sell Nylon and Dyneema slings.