I've never seen one, but it can't be too hard; it's just plumbing. I'd ask first why you want a water filter; unless you went with a reverse osmosis unit, it pretty much just takes out the big chunks anyway. Why not just use a filter on the incoming hose connection and thus cover the bathroom sink as well?
"Between what matters and what seems to matter, how should the world we know judge wisely?" - E.C. Bentley, Trent's Last Case