As Ryocrits mention there are no official fansites with merch products. It seems there was something happening in 2008(Link below)1 but it does not show any game related shirts/items anymore.
However, I found few companies which specialise in personalised products and they have some products on sale:
https://www.redbubble.com/shop/tibia
https://www.teepublic.com/tibia-merchandise
I would recommend to actually go with companies which are doing personalised products and compose something by yourself;-)
Fabio_Z from Reddit has his website- https://www.lux3d.me/tibia ( you will be able to see the products without visiting the website in here https://www.reddit.com/user/fabio_z/ - that is in case you do not feel safe to visit it and still curious).
1. https://www.tibia.com/news/?subtopic=newsarchive&id=750