I found one for an Arduino Uno for ~$2 at a local store and then snapped off one of the panels to expose the micro-USB connector:
It’s not pretty, I can’t reach the Reset button, for instance, but it’ll protect my HiFive1 when I chuck it in my backpack. Having said that, I recently had a work project which needed a prototype case made so we went down the 3D printing route. It turned out finding a local printer was very easy (offer may not be valid if you live outside a large urban area):
It’s also easy to find open-source 3D designs for Arduino cases and modifying one for the connector layout of the HiFive1 could be a fun side project.