M, if it is similar in size to the smaller automobile ones, then you can safely get one from Midas.
You might have to swop over the springy mounting bracket, though. That will merely need a spanner and plasters for two skinned knuckles... unless you can flutter your eyelids and get unsuspecting hero-worshipper to skin his knuckles for you.
Bottom line is... nothing special about the bike hooter. Just the size is important, that's all. It must fit. They are all 12v and all have a similar frequency range.