Search for "usb c cable 100w 3m" or 10ft if you are from the US on amazon or similar, there are many available - just select one with good reviews. 100w is of course overkill for your 30W MBA, but it will ensure a high quality cable. Alternatively, I hear good stuff about Aukey and Anker cables and you can save some money by choosing a 60W cable.
Another alternative is getting an extension cord for your charger, Apple's chargers even use a standard electric connector for the cable, so you can find one for like a buck. Obviously original Apple's cable will look nicer.