Via the Box Office page or your Customer-Facing Booking Flow.
If you want to sell Products as upsells, customers/box office staff can choose the Event and tickets first in the booking flow and then you can add Products on the next page.
You can also sell a Product as a post-ticket purchase upsell.
If you are selling a Product by itself, you just need to select the Standalone option on the Product info page. You can also create a Product-themed URL for your website booking flow.