As has been suggested most if not all hub dynamos will do the trick. For me it comes down to usage versus price versus reliability. Based on consideration of those three criteria I have two dynamos: a SON 28 and a Shimano Deore XT DH-T785. I had considered a Shutter Precision for a third setup but have seen a few negative reports on them and given the options will now pass.
For charging purposes I use a
PedalPower+ Super-i-Cable but I have had a few issues with them. I am hoping my latest replacement unit will be sweet.
If you go with a dynamo hub you will need to either rebuild your existing front wheel or build a new one. As to price well that really depends on your options and where you purchase it from. Personally I buy mine from Germany, from
http://starbike.com as they also build the wheel for me pricing is very good.
Andrew