Some merchandise is mixed with the zines on the zines pages.
For many Fandoms, where there is merchandise available, there is a link at the top of the Fanzine page and from the list of fandoms. The merchandise page for a fandom, has two parts, at the top are the designs which are available and lower down the items available. For most things, choose a design, and then decide on the item you want. For many items there are options such as T shirt size, colour etc. Fill in the options for the item you want then click on the button for that item.