A good site is Teachers Pay Teachers - it's technically for teachers but they often have freebies. The flashcards are cheap - $1 - $2 and even if you don't buy you can definitely get a lot of ideas
You can try this site! http://www.brillbaby.com/free-download/flash-card-printouts.php It has animals, fruits, colors, etc.
This link might help too:http://www.makingmontessoriours.com/2011/09/complete-list-of-montessori-materials.html