The Tôkyô National Diet Library Digital Collections
The Tôkyô NDL Digital Collections contain a vast number of old Japanese prints, many of which are available as IIIF-documents and thus compatible with DemiScript.
The Harvard Library Digital Collections
With over 6 million digitized objects, the HLDC offers a wide variety of IIIF documents for perusal.
The British Library
The British Library has over 50k IIIF documents in its extensive catalogue. They can be looked up using the pre-entered search string baked into the link below. Just click to search for IIIF documents in their catalogue.
The David Ramsey Map Collection
With over 150,000 maps, the collection focuses on rare 16th through 21st century maps of North and South America, as well as maps of the World, Asia, Africa, Europe, and Oceania. These maps all come with DemiScript compatible IIIF manifests, which may be a bit difficult to find at a first glance, so here is an image that shows where to look for the manifest links:
The Digital Bodleian project has made portions of the Bodleian library collection available online, offering nearly 15k digitized documents with IIIF manifests.