If you are looking for a specific file, I highly recommend conducting a detailed Google search such as "astronomy nomenclature cards" or "parts of a tree 3 part cards". You'll be amazed at what is available for free.
Montessori Print Shop Offers both free and for pay downloads. This is generally my first stop.
Montessori For Everyone Another fantastic shop that offers both free and for pay downloads.
What sites do you use for your Montessori printed materials, free or for pay?