If you handle a lot of damaged books like ex-libs, or books where the ultra paranoid have torn out their name, you are going to have to replace an endpaper from time to time. Unless you want to run out and buy a new old book that matches your repair perfectly you may want to build up a collection of endpapers from deceased books. We all have our own idea what makes a good dead book, for the sake of argument let’s just agree that a deceased book is one where the parts are more valuable than the whole.
Once you have harvested the book parts, you can save the endpapers and blank flyleaves in one file drawer. Group them by width and height, let the age, color and material take care of themselves. That way when trying to match, you start with folder nearest the original size and work your way up through the sizes. Remember large epigraph or dedication pages make good cutdowns for smaller books. Some of the more ambitious repairers will also save skived book cloth, boards, labels, ribbons whatever can be salvaged.
If you STILL can’t match the endpaper, you can always take the rear free endpaper and flip it over and make it the front free endpaper. (a missing rear endpaper is much less noticable)
Here endeth the lesson.