Apparently there's a series going from the Slovenska Akademija Znanosti in Umetnosti, Ljubljana. Holdings in the US are very scanty. I don't know if it's complete or not.
The Opus Musicum is 6 volumes of Denkmäler der Tonkunst in Österreich, and is a goodly chunk of the motets, and pretty common in academic libraries.
He's a special case in that Grove doesn't provide a detailed worklist, but you can peruse titles at the CPDL Opus Musicum page, which also has a link to the BSB partbook viewer for vol. 4.
The Slovenian Opera are nice paperback volumes, and a friend was able to get the consulate to donate multiple copies for an all-Slovene concert here a few years ago. They're part of a series Monumenta artis musicae Sloveniae.
Between 1985 and 1996, Slovenian Academy of Sciences and Arts published his works in twenty volumes as part of a series Monumenta artis musicæ Sloveniæ, as Richard mentioned earlier. The editor of the collections was musicologist Edo Škulj. I don't know if the collection is available anywhere in digital format, but it is available in many libraries in Slovenia.
Depends on where you do live. In the US I'd try Cantus' link before the usual music dealers, who tend to have hefty surcharges on overseas orders. And of course if you have Slovenian friends…
Maybe you can try to contact the SAZU Institute of Musicology (muz_inst@zrc-sazu.si), maybe they even have something in digital format or can deliver by mail.
To participate in the discussions on Catholic church music, sign in or register as a forum member, The forum is a project of the Church Music Association of America.