The first thing you need is to be able to describe the crest. This link
https://americancollegeofheraldry.org/wp-content/uploads/2024/07/HERALDIC-MEANINGS.pdf
will help you work out the items that might be included – just look up something identifiable in the design (a tree, an eagle) and make a list of the components of the crest.
With that list in mind, you can move on to describing how that thing is presented: is the tree in full leaf or is it leaning at an angle? Does the eagle have spread wings or is it tearing at a piece of meat? This link
https://historylearning.com/medieval-england/heraldic-terms/
will allow you to get to descriptions (possibly in Latin or Old French) to add to the words to your list. You now have a description of your crest: there are several online resources that can help you to identify the family that used buttons with the crest you can describe. I use these digital books all the time, even though they are very old!
Alexander Deuchar’s 1817 book British Crests: the Crests and Mottos of the Families of Great Britain and Ireland is available as a Google book as well as in archive.org: use these links or copy the title and search for the book.
https://archive.org/details/britishcrestscon02deuc/page/n5/mode/2up
John Peter Elven’s The Book of Family Crests comprises two volumes and was published in 1854. It is still available as a physical copy, but (August 2026) costs around £150… It is available online from several different sources.
The Wellcome Foundation’s website:
https://wellcomecollection.org/works/sgtyzh87/items?canvas=5
Google books: the whole book is there, but you have to download the pdf (use the cog, top right) to see it.
https://books.google.pt/books/about/The_Book_of_Family_Crests.html?id=mukWAAAAYAAJ&redir_esc=y
The Internet Archive:
https://archive.org/details/bookoffamilycres00elve
The Hathi Trust: again, you have to click a link (scroll down to see it) to read each of the two volumes.
https://catalog.hathitrust.org/Record/008685857
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