I've found/bought the squire, knight and vampiric versions of all the equipment, but I cannot find any more blueprints. Where do I find the rest of them?
More blueprints are only available (or at least the spawn rate is low) in harder areas of the castle.
Not all equipment is available in the first play through of the game. You need to be on NG+ or beyond to find all of the best blueprints.
From my experience, in NG++ I was able to find the last tier of gear fairly quickly.
If you are having difficulty finding blueprints, try hunting mini-bosses, they have a higher chance of awarding chests that contain blue prints. You could also visit the Spiritual Shrine and hope for a Calypso's Compass