EmbLibrary has a great selection, available in sets or as singles for $1. Maybe too many choices, ha ha ha.
I also think we have some here on designs by cuties. If all else fails, use the button hole program on your machine , if it has one.Also check your buttons to see what size you need. Love, Kay
The size depends on the fabric and where it is placed. Is it for the back of the bib to fasten it on the baby? Around 1/2 inch would work.
Here are some freebie links. Some of them you need to scroll to the bottom of the page.