Usually they just post it on their social media platforms. Now if you want them, either it may be available in high resolution on their website or you can download the image and use something like super resolution to increase the image size.

Now if you don’t know what is super resolution just let me know I’ll increase the resolution and upload the image somewhere so that you can download it.

Edit : There are some older posters for purchase on the official F1 site, but I feel like they are horribly expensive just like everythingelse F1 related.

https://f1store.formula1.com/en/collectibles-and-memorabilia-posters/d-19227704-4562110217+z-97-1194366222?sortOption=NewestArrivals&pageSize=1