Amazon seems to have a wide range of them. Some keywords to try:
"glass eyes"
"toy eyes"
"20mm <or whatever size you want> eyes"
Some have free overnight delivery. If you want to be sure they arrive
quickly, look for "prime" in the listing--you don't have to use Prime,
but if they're listed as "prime" they generally are in stock in an
Amazon warehouse rather than coming from a third party who might not
be as efficient on shipping.