If you have a relatively modern Shimano dynamo hub, it should be specified for the "standard" 3 watts. If so, then you can use any B&M dynamo light. You can order them cheap from bike-discount.de.

However, B&M lights are usually quite dependable. So, unless you damaged the light or submerged it, I'm wondering if the light itself isn't the problem? Maybe it's a loose or frayed wire somewhere?