Zwv 19, 20, 21

It doesn't seem that full scores are freely available. No one seems to have finished any attempt at creating and sharing a public domain score of the works. Looking at the manuscripts it's understandable why. You can attempt to buy the scores for outrageous prices; if it's for scientific work, maybe you can get some funding for this?

You might as well just use the manuscripts, they aren't too bad to read though I suppose that depends on what your research is exactly. I think at least number 21 is available to purchase somewhere in Europe but for large sums of money unfortunately.

They are quite difficult to find at an affordable price, and hopefully the situation will improve over the next years. As Rik1 said, the best and cheapest option at the moment is to get stuck into the SLUB manuscripts. The higher-resolution files are all available on the SLUB digitale website. IMSLP has only lower-res at the moment.

Some other tips:

ZWV 20 - Ensemble Inégal have parts for instruments (not vocal) available to purchase. But they don't have the full score on sale. To find that, it is worth searching around online for sheet-music shops, most of which are based in Germany. Breitkopf have the only edition, which was done at the end of the 1980s, it has this article number: EBSON305. I remember when I searched for this back in late summer 2015, it was priced on various sites at €159.00...

ZWV 19 - There have been two groups who have done this Mass very recently, perhaps you can ask them for advice?: Le voix de montagne, from Montreal, Canada; Also Bach Collegium Paris, whose concert is on 11 June. It would be nice if you knew some French!
Breitkopf have the score here, once again, for a cost.

ZWV 21 - It is worth contacting Ensemble Inégal. Also, Breitopf have a piano/vocal score.
If you are not under a tight schedule and are very patient, a few of us on the forum are a few movements into creating a full score.

All these scores has been published at Breitkopf as "Erbe deutscher Musik" Vol.93 (ZWV 19), Vol.100 (ZWV 20 + ZWV 151) and
Vol.101 (ZWV 21+ ZWV 152). One may order these volumes in a scientific library, perhaps via inter-library loan.