When purchasing on amazon it will specify if the blank is unfused. Amazon has a generous returns feature, If they deliver you a mislabeled product you can return them, I would recommend wiping any data you write onto them first though haha.
you can also source blanks direct from the manufacturer through platforms such as alibaba. I wouldn't recommend factory to consumer services to start out with as it can be time consuming and costly to find a worthwhile manufacturer, if you go down this route i would recommend getting them to send you a sample first to test the viability of the product. The advantage of using services such as alibaba can be great however, you can get a lot of cards at once to reduce risk of suspicion (as chinese factories don't care for what business you're in) and for much cheaper then if you got cards in small batches.