Do you know how to get Pill Bugs If not, this is how.
Get a shovel. Now, hit any rock with the shovel. If you don't see some kind of Black Dot popping out of the rock, try another rock until you do see one pop out. When one does pop out of a rock, get out your Bug Catching Net. Use it to catch that Black Dot. When you do, you'll find out that it is a Pill Bug! This can help you fill up your Bug Encyclopedia.
Now, you can donate it to the museum, sell it to Tom Nook for some bells, or just keep it for yourself.