This goes against the grain of many sales people, but sometimes it is best to link to another site that does offer the solution. Sure, you will lose this sale, but you would have lost it anyway, and by giving the user a useful referral, you will have won credibility points. Just as important, you will have secured a position in the user's mind as the place to start the next time they go looking for solutions.
If I'm in a store and they don't have what I want, I always ask where they think I can get it. Some don't know, some seem to know, but won't tell! Thus falling into the trap outlined by Jakob Nielsen above. Sometimes he seems to be stating the obvious, but life often proves that stating the obvious is necessary and sometimes the obvious isn't that obvious unless someone states it! :-)
"Childhood is short and maturity is forever." - Calvin.