Fluke supply spare parts for their equipment. As to cost … no idea, you would have to ask them. I would expect such a part to cost around $10 + taxes and shipping.
If you do not want to spend money on the unit, why not find a longer screw and washer to secure the battery cover or find a screw with a larger diameter head ? If you use a large diameter countersunk head, the screw will not protrude too much from the rear. Does the yellow boot not hold the battery compartment cover in place ?
If all else fails….. insulation tape !