Without question, the most important accessory you can purchase for a stringed instrument is a case or bag.  Many student or intermediate basses can be purchased as part of an outfit that would include a bag that is usually consistent with the quality of the instrument. These cases generally offer very adequate protection and durability at a economical price. 

Padded bass bags are standard cases for 3/4 (full) or 1/2 size basses.  These bags feature heavy-duty nylon oxford pack-cloth exteriors, reinforced bridge padding, accessory and sheet music pockets, a bow pocket (usually large enough to fit a bow case) and padded shoulder straps.  Bass bags are foam padded, with padding thickness ranging from 5mm to 25mm. 

There are great bass bags on the market today that offer an easy way to transport your large instrument.  Some offer backpack style carrying to distribute the load over your back, while others come with rubber-coated wheels.