Dulux Mysterious Teal is your best bet, mate. At ΔE 1.4 from the original it's a very close match — that's well under the 2.5 threshold where most people would struggle to tell the two apart on a wall. With an LRV of 9.5 it sits in the same deep, light-swallowing register as Mockingbird, so the character carries over. You get the same dark teal drama at a fraction of the cost.
If you'd rather stay a touch more premium but still under Paint & Paper Library money, Crown Endeavour lands at ΔE 2.9 with an LRV of 9.8. That's just outside the "very close" band, so you'll notice a slight shift if you put the two side by side — but on its own in a room, nobody's going to clock it. Crown's trade availability also makes it easy to get hold of in volume.
COAT The Drink is the loosest of the three at ΔE 3.6 and a darker LRV of 7.4 — it reads deeper and a little moodier than Mockingbird. I wouldn't call it a true dupe, but if you fancy something in the same family with COAT's lovely low-VOC eco credentials and brilliant coverage, it's worth a sample.
The big "but what about" here: a colour match on screen or on paper is never the whole story. Deep teals like this are heavily affected by finish and light. A matt estate-style finish will read inkier and softer; a fuller eggshell will pull more of the green-blue out. Mysterious Teal and Mockingbird may match on paper but a different sheen level will change how they feel.
So — order sample pots of Mysterious Teal and Endeavour, paint two coats onto A2 lining paper, and live with them on the actual wall for a couple of days. Look at them in morning and evening light before you commit. Nine times out of ten Mysterious Teal will be the one, and you'll have saved a decent chunk doing it.