I may be overlooking something, but I’d think you might be able to substitute an LM358P ($0.41 at Mouser) for the NJM2732D ($2.58)… yes? No? Worth a try?
As for the MCP4726, apparently exists only in SMT form. @HAGIWO evidently used a breakout board “purchased from Akizuki”. Might be out of luck if you want a through hole build, but per discussion on reddit Hagiwo’s quantizer circuit, at least, could probably be converted to work with an MCP4822 — would need code changes, though.