How do I cite ebooks without page numbers?
You are not providing page numbers with citations for their own sake, but to help readers to locate the cited passage, e.g., if they want to verify it or see it in context. (Therefore page numbers are already diminished in their usefulness for regular books as soon as there are two editions with significantly different paging.)

The arguably easiest way to locate a verbatim quote in an e-book is to just feed a few words into a full-text search. Thus giving page numbers or similar information has no purpose anymore. However, you could ease finding the location of the quote in a classical book by giving edition-independent location information, such as chapter and section numbers.

Note that this is a “utilitaristic” approach to citations. A relevant reader of your publication (e.g., a supervisor or reviewer) might have a “dogmatic” view on such things and thus require page numbers or similar for their own sake.