Actually, since you are familiar with calibre it will be easier to use it than Sigil for this particular task.

First, look up the ISBN number for the book, Amazon is a good resource for this. (Note: Calibre should be able to find and download the cover without an ISBN number, but I've had problems with it in the past and found that it does better when you have the ISBN number.)

Click Edit Metadata in calibre. In the Edit Metadata Information window, type in the ISBN number in the ISBN field, then click the Download Cover button. Calibre should be able to find and download the cover. Once the cover is downloaded and displayed, click the Okay button to save the information.

Then convert the book and sideload it.