Alright, practical rundown: the official xkcd merchandise and prints come from the creator’s own store
linked on the xkcd site. You can browse comics and shop via the 'Store' link on xkcd.com (often redirecting to store.xkcd.com). The shop lists individual comic prints (many comics are offered as single-sheet prints or posters), apparel with classic panels, mugs, and other small goods. Buying there ensures you’re supporting the cartoonist directly and getting officially sanctioned items.
If you’re looking to buy a print of a particular strip, search the store for prints or the comic number and select the size and finish you prefer. The site usually notes paper quality and available dimensions. It’s also handy to remember the Creative Commons (noncommercial) license attached to most xkcd strips — cool for sharing or printing a copy for personal use, but not for selling. For collectors, the store sometimes has limited editions; for everyday buyers, it’s the steady source of new stock, reprints, and restocks. Payments, shipping, and returns follow the store’s policy, so check those at checkout.
Personally, I’ve bought a couple of prints and a mug there; everything arrived well-packaged and felt legitimately well-made. It’s my go-to when I want something that’s both funny and officially sanctioned.