Loads of versions of cubase came out for the ST - that latest was Cubase Score back in 1995. eBay is about the best place to look for a copy of the program - you can still get it retail from some places but it's likely to be expensive.
Try also www.systemssolutions.co.uk
www.atari-workshop.co.uk
and www.atari.st is a great resource for games.
I can't think of a better place to go for help than right here - the people on this site really know what they're talking about. I just pretend ;-)