I'm not aware of any currently-available aftermarket plastic blocks. I do recall getting hold of some quite a few years ago; they were made by one of the European ship model companies (Billing, I think). The plastic was a slightly flexible, shiny, cream-colored substance; paint wouldn't stick well to it. They were nice, clean castings, but awkward to work with - and far too big for the Heller Preussen.
The problem with making decent blocks in plastic is that styrene, of the type normally used in plastic kits, has to be injected into a rigid, steel mold - and a two-piece rigid mold, by definition, can't produce a part with a hole in it and a groove around it. (Presumably that's why that European firm used flexible plastic: it could flex enough for the finished block to be removed from the mold.) Many years ago the geniuses at the Japanese company Imai figured out how to do it, apparently using a multi-piece "slide mold." If plastic blocks made with that technology were available in quantity I'd buy them by the gross. Unfortunately, though, they only seem to be obtainable in the few old Imai kits that are still on the market under other labels. The Academy (ex-Imai) "Roman Warship" contains about a dozen - along with some equally beautiful deadeyes.
The alternatives are wood and metal. My personal favorites are the cast britannia metal ones from Bluejacket (www.bluejacketinc.com). They're nice, accurately-shaped castings, available in a huge variety of styles and sizes. They do require some cleaning up with a small file, and they obviously need to be painted (or "blackened," with a liquid called "Pewter Black" that Bluejacket also sells). The smallest Bluejacket blocks are, to my knowledge, the smallest available from anybody - and for the Heller Preussen you're only going to be interested in the smallest two or three sizes.
For wood blocks, a good source is Model Expo (www.modelexpoonline.com). That company sells quite a variety of rigging fittings from various European manufacturers. They vary somewhat in quality, and I suspect virtually all of them are too big for this particular project.
Hope that helps a little. Good luck.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.