Sorry to hear you failed that part of the test. Is that with a TOC or Network Rail?
I'm not aware of anything online to 'help', although something might be available commercially for potential drivers (there seems to be a lot of money to be made...!). However, the purpose of these tests is to establish whether candidates have the right 'non-technical skills' (concentration, ability to multi-task and all that), not how much they've practiced beforehand. They're designed so that, for the most part, you can either do it or you can't.
You could, I'm sure, set something up yourself with a willing helper and some means of producing two distinct tones!
Did you find that part of the test (and indeed the rest of it) particularly difficult, or do you feel that you did well but just missed out on a pass?