The majority of garden room suppliers don't have a showroom. Instead, they have arrangements with past customers to enable new customers to visit their garden room and have a look around.
The beauty of this arrangement is that your chosen supplier probably has a garden room you can view, within an hour or so from your home.
This is a valuable experience as you can spend time in the garden room and also ask the owner questions about their building, and the experience of buying/building it.
To arrange a visit to view a local garden room, contact your chosen supplier and ask them if they can arrange a viewing for you.