Hello Cathryn,
Welcome to Microsoft Community.
According to your description, the Type Cover you have is made of Alcantara material. The material usually has the ability to spill and absorption-resistant:
But if this is a used device, then it may not be as good as a brand new Surface, after all, the Surface Pro 6 was released in 2018, six years ago.
However, the Type Cover suitable for Surface Pro 6 is divided into two materials, one is Alcantara material, and the other is conventional plastic material:
Microsoft Surface Pro Type Cover | Surface Pro Keyboard - Microsoft Store
However, you will notice that the official store marks it as out of stock, and you can talk to a sales representative through the chat window at the lower right corner of the page to ask if there is a replenishment plan.
Or maybe you can head over to Amazon to see if any dealers are still selling the Type Cover for the Surface Pro 6, and be sure to make it clear that you want to buy the plastic Type Cover, not the Alcantara Type Cover.
As for third-party keyboards, I'm afraid none of them will meet your needs, because Surface's ports are special.
Good luck to you.
Best Regards,
Mitchell | Microsoft Community Support Specialist