Sorry cannot help you with this, but if you're not completely satisfied with the light, now might be a good time to consider a replacement. A lot of people are phasing out fluorescent lights for other types of fixtures these days.

Good luck, you most likely will not find it.

You might consider replacing that old florescent fixture with an LED wraparound.

Having recently been through similar, it's not worth wasting the time looking for the specific part and cover. You'll do much better to simply replace the whole thing, likely with some kind of LED. You can go as cheap or expensive as you like, and some are sold with better water/steam protection.