You can create a segment of customers based off of a particular product that has been purchased. You can do this in the Basic or Advanced Segmentation builder (Lists>New>Segment):
For example, in the basic segmentation builder, you can segment on "Purchased Product Names" in the drop down menu.
Select the "is in" if you want to quick search:
From here, you can search or scroll down in the list!
When you have selected all of the products you would like to segment on, you can then "Update Count" to see how many subscribers would be in your segment.
Remember to give your segment a name, and then you can SAVE your segment!