if they are like funny shirts, I would use Facebook/Instagram ads.
I know about qwertee.com/ from their banners few months ago on this social networks and I have ordered 10+ shirts already already (their quality is bad, but I cannot say I have clicked on a banner anywhere else and buying after that).
Let me explain - if I am reading something I will never click on banner I am reading, so you have to market somewhere were people (even programmers) just scroll and do not read !
However keep in mind in many countries winter is coming, so do not expect a lot of t-shirt sales. In the spring its a lot different, so maybe it is wiser to wait for the huge marketing campaign.
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