There are several models out there. Or you can build one yourself (see website below - about a 1/3 the way down the page you'll find the lily pruner). Here are some examples of ones you can purchase:
Click here - Lilypons
Click here - Laguna
There are one or two other models out there. I have one from Paradise water gardens. Fiskars makes a "pole stick" - about 5 feet long but it is a regular tree pruner which is too large, I think, for trimming single lily pads. The biggest problem with these things is tracing the dead pad down to the main plant (same for clipping the flower stems)...sometimes you'll find yourself cutting healthy pads off your plant!
---David

Here is a link that might be useful: Several do-it-yourself projects for your pond