There certainly are for iPods, but other MP3 players don't really have the sales muscle to warrant a big 3rd party accessory base.
If the MP3 player uses a standard mini-USB socket it may be possible to use a car charger intended for a phone that uses the same plug -- but these don't always work with every MP3 players. For example -- I have a Sony and an LG MP3 player and the Sony will charge from a Blackberry phone mains charger but the LG appears to charge but doesn't. It may be down to the software in the player.