I've actually been advised by a friend from Hong Kong that a good substitute that is generally available from good chinese grocers is called "Ve-Tsin" or "Vetsin". It comes in a pretty gold and blue tin, and is exactly the same thing as Mikku.
As per the previous post, it is basically MSG mixed with table salt. If you are worried about it, leave it out, but it's a naturally occurring molecule in foods such as seaweed, hence the japanese connection. I've used Ve Tsin, and it's really great.