Wanted : 50mm ring for sentinel

Hey all, I have a tickleberry sentinel, though I think it's a copy as I got it from lovehoney for £40, unfortunately the provided ring doesn't fit. I need the 50 mm one, though I'm loathe to pay £50 (extreme restraints) or £70 (tickleberry) for a hunk of shiny metal, especially as i'm soon to be a poor student again! Does anyone have one spare/for sale? Or, does anyone know where I could get one for a decent price?