[LAU] Solo stringed instrument sounds

I am in the process of creating a MIDI version of some string quartet music.  So far I have been using VPO solo strings sounds, but these are not very satisfactory even after editing the .sfz files to the best of my (limited) ability.  VPO has some great sounds for string sections no doubt, but for solo instruments it is not so good. I downloaded an sf2 version of some solo string sounds and converted them to sfz, but found that they were very synthetic sounding. I have also downloaded the Philharmonia Orchestra samples.  These are fairly comprehensive and sound good, but unfortunately the samples themselves are very short (typically only 1 or 2 seconds long), and I haven't been able to make satisfactory loops with them. Can anyone suggest where I might find some good quality solo string sounds?  Ideally, as well as normal bowed and pizzicato sounds, I would like natural and artificial harmonics and open (i.e. unstopped) string sounds.