They can be quite costly, a few hundred bucks to start and they go up in price the better they are. There are a few that are said to be pretty good for the price. Check out Konica Minolta Dimage Duelscan...or something like that.
Just be aware that some scanners only scan 35 mm while other will scan medium format and large format negatives as well. Epson makes great scanners too but they wear out pretty fast- i ve had one for about 3 years and it's finally causing newton rings in negative scans. microtek(sp?) has some reasonably priced ones that handle all formats and slides.