I have used GHS Precision flats and bought the B string seprately to use on my fretless five. The old school sounding strings were most excellent of the bass and were about the around the $30 dollar mark, perhaps a dollar or two more.
Carvin also sells a 5 string set on their website that is not that expensive, and appears to be to be re-branded GHS mediums with the .128 gauge B that you usually buy seperately on the GHS label for a little less. Shipping is higher, however, so you will want to buy more than one set or thing.
Carvin.com :: FW50