Olá @Filipa Raquel,
there is no option to buy ServiceNow merch unless you attend some of the meetings, there you might have a chance to get some swag but not online.
Other chance is to attend ServiceNow challenges (usually september/october) where there is a chance to win something when you develop your app, last time it was a mug for example.
Last but not least, I saw some shirts being sold on amazon and ebay but I guess these shirts are ones that somebody got on a conference and now they are selling it, which means you cannot select size and nor logo etc...
And if you are feeling lucky, go complete this class and you might win ServiceNow merch:
https://learning.servicenow.com/lxp/en/now-intelligence/ai-essentials?id=learning_path_prev&path_id=...
