Hello Marie,
To get a gear made will cost an absolute fortune, I am not sure when your machine was made but most companies keep spares for up to 10 years, failing that if you contact www.sewingonline.co.uk/instructions you will find that he is always breaking Berninas for spare parts, you may be lucky, this is the only person I know.
Ken
drivewithken@blueyonder.co.uk, October 2008