I work in a place that makes delicious herbal coffee, and sometimes there is a lot left. It would be great to be able to freeze-dry it in small batches and be able to use it later!

Does a machine like that exist for small batches, basically for household usage? The medical machines I saw online are expensive! Is there something more affordable?

This depends on your cooling agent, needless to say this isn’t portable not matter how you go about it, but a large perltier and proper heatsinks and large fans can do just about everything. There is a great webistie that explains the process called howstuffworks.com and all you need to know is how you want to cool the specimen (in yoru case coffee)