The second string quartet in D seems to be at British Library at Yorkshire's Library Reference Collections (www.worldcat.org is a reasonably good resource for locating these things, though it's well-known so I mention it just in case.) His opera Sanna is in vocal score at Arizona State University. U California Berkeley has "Die letzten Tage von Thule". No idea who has full scores of the operas; presumably his estate, or possibly the publisher of the vocal scores. Possibly.
(The C minor quartet has recently been republished - 1997 - by Amadeus-Verlag, I think, making it easily his most accessible score anyway.)
(The C minor quartet has recently been republished - 1997 - by Amadeus-Verlag, I think, making it easily his most accessible score anyway.)