I wear both hats but I am primarily a collector so I would like to make some suggestions and encourage your participation by sending additional thoughts and comments to
Find A Hand Bookbinder
Bookbinders are hard wired pac rats. They often save old boards and keep bookplates when the customer does not want them.You should be able to find one or two . If you need help ask your local antiquarian bookseller for a name. .My experience has been that bookbinders generally are very helpful.
Antiquarian and Used Booksellers.
Make yourself known to your local bookseller(s) and ask them to keep you in mind if they get a book (within your price range) which has an impressive bookplate.While you are at it ask if they have a box with detached boards or ephemera.From time to time I have purchased some exceptional bookplates that way.
Join The American and or The English Bookplate Societies..There are links to both organizations in the column on the left side of the blog.
Local Book Sales and Book Shows
This one is a no brainer.You need to get on their mailing list
Tomorrow if all goes according to schedule
I'll be in New York City for the Herring Festival.
If you plan to be in New York City this month and you like Herring here is a link.