Direct answer: Estate sales, library used-book sales, and specialist marketplaces like AbeBooks (searchable by "children's" category and illustrator name) are the three strongest sources for vintage children's books, generally outperforming general secondhand bookstores for depth of selection.
Why vintage children's books are trickier to source
Children's books historically had lower print runs and much higher rates of physical wear (they were, after all, made to be handled by children), which makes well-preserved vintage copies genuinely scarcer than adult fiction from the same era.
Estate sales and library sales
Multi-generational family collections liquidated through estate sales frequently include children's books that were kept in better condition than average, simply because they were stored rather than actively used for years. Library book sales, similarly, often surface donated personal collections in solid condition.
Searching by illustrator, not just author
Vintage children's books are often collected and searched by illustrator as much as by author or title — searching a specific illustrator's name on AbeBooks or eBay frequently surfaces titles a general title search would miss entirely.
Reproduction and reprint editions
Some beloved vintage children's titles have been reissued in facsimile or lightly updated reprint editions, which is a much cheaper way to read (or gift) the story, though collectors specifically seeking an original printing should confirm which edition a listing actually is.
Are vintage children's books always more expensive than adult vintage books?
Not always, but well-preserved, illustrator-notable copies can command a premium precisely because survival rates in good condition are lower.
Is it safe to read an actual vintage copy with young kids?
Many collectors keep fragile vintage originals for display and buy an affordable reprint for actual reading use with children.
How do I find out who illustrated an old children's book?
The title page or copyright page usually credits the illustrator; if unclear, a library catalog record or Open Library listing will typically list it.