Question about Xiaomi Mi Airdots Pro

The original silicone attachments are short so that they fit into the charging case, cutting them off is easy, especially if you have a whole bag full to practice with ;)

Do you mean the ones that are available here at Digitec? That would be the Comply 400 series (4mm inner diameter).
But since they are senselessly overpriced and too long for the charging case (you have to shorten them with scissors anyway), I advise you to import them from China.
Foreign links are not allowed, look for example at Aliexpress for memory foam with an inner diameter of 3-4.5mm, you can get a whole bag of them for 1-2$.