Apparently someone needs to do your work while you are not there. This won't happen by itself, so someone has to find someone to do this. Of course your boss can ask you to find that replacement. That would be part of your job, so when you go to work tomorrow morning you ask, during your working hours, anyone who might replace you.
However, since sorting out work schedules is probably not your job, you might not succeed. That's bad luck for your boss. Whether you find a replacement or not, you go to jury duty, and there's nothing your boss can do about it.
PS. You could look at what mhoran_psprep quoted, and whether you think telling you to find a replacement is "harassment". If the boss expects you to do it in your own time it surely is. If he complains about you not finding a replacement, when finding replacements isn't your job, that could be harassment as well. (I suppose if you worked in HR, then finding replacements for people going on jury duty could be part of your job, and it would still be part of your job if the person on jury duty happened to be yourself. )