No reason you can't have M30 A4 stainless fasteners, you just won't find them in any stockists that's all. As the size is uncommon, it would be a special job for a manufacturer.
Are you looking for a particular length? Full or Partial Thread? And are we talking a hex bolt? As Yates said, this is a standard metric size but the A4 makes the availability a lot more challenging. But before I would go so far as to say it isn't stock anywhere I would need to know the above.