I want to get a reprint of an old apple photo book. How do I find this?

Apple does no longer print books at all since more than a year.

If you can find a PDF file of your old book, you can upload this PDF file to one of the third-party services, that can still print Apple photo books.

Do you still have the photo library, where you created the book? Did you create the book in Photos or in iPhoto?

As leonié has suggested do you have a PDF file of your book or are you able to create a PDF file for that book?  If so there's a 3rd party book provider which used to do the printing for Apple as a sub-contractor, Mimeo, that will take PDF files created by iPhoto and Photos and print and bind books and calendars. 

Also you can upload the PDF file to PrestoPhotos which can also print and bind books and calendars from iPhoto or Photos pdf files.  They've been doing it for some time now.

Use the PrestoPhotos or Mimeo links above for direct access to the PDF file uploading and book/calendar ordering page for each site.