Alternative thought. I had a similar, but lesser need (went for a 360ppr encoder), and slaved a board inline which on start-up established a synthetic 'Z' pulse (basically, counts the quadrature signals continually and generates a Z pulse once in 360 etc), passing the original A/B input through to the output. One advantage was in establishing this I was able to use RS485 line drivers for the new A/B/Z output drivers which were pretty much impedance matched to the differential inputs on the Mesa 7i76E board I was interfacing to. I'd hazard a guess for any incremental encoder would do just the same.

Just an alternative if you struggle to find an affordable encoder with a Z output.