When I first started planning my perfume classes, I decided that for the fundamental series, I will offer the option of using common spices and fruits as study aids. My belief is that anything can be useful to training our sense of smell, as long as we follow the proper technique, smell consciously and practice regularly. You can create a box of spices and study them, noting their different nuances. Even after a week of diligently smelling your spices, you will find your sense of smell much sharper.
Once you learn the basics, you can consider investing into a smelling kit. I researched a few reputable options, compare them and made the guide below. Some of the products I mention are available in the US, others in the EU. Either way, I hope to give you a few choices. And to repeat, these kits are not necessary when you first start out with your perfume explorations, but they become useful if you devote more time to unraveling notes and accords.