Phone up Edition Peters or someone. They're bound to be able to either send you a copy or give you the details of a publisher who'll be able to.
Rachmaninoff is a pretty famous composer. His complete works should be available under most general music publicists.
EDIT: I doubt you'll find much of his music online, as he only died in 1943. This means that most of his music shouldn't yet be in the public domain as far as I'm aware, at least certainly not in the UK. If you go to mutopia.com, a website full of public domain music, you'll see that the latest composer whose music they can share legally died in 1885.