Hunter Dunn is a gorgeous deep, slightly grey-leaning forest green — the kind of colour that does cracking work on panelling, a study, or a moody kitchen. But Paint & Paper Library sits at the premium end, so I understand wanting a closer-to-earth price.
Your best bet is Dulux Woodland Fern 1 (LRV 7), which lands at ΔE 3.4 from the original. That's not imperceptible — anything under 1 is — but at 3.4 you're in genuinely close territory, and once it's on the wall in your own light you'd struggle to tell them apart. Dulux Trade in a good eggshell or matt will give you durability and a price that's a fair bit kinder.
If you fancy something with eco credentials and lovely depth, COAT's Ditch the Tie (LRV 8.1) sits at ΔE 3.6 — practically neck and neck with the Dulux. COAT's water-based, low-VOC finishes are a pleasure to roll and the colour holds up beautifully in low light.
I'd steer you away from Crown's Woodland Wanderer (LRV 10.7) for matching purposes — at ΔE 7 it drifts noticeably lighter and greener, so it reads as a different colour rather than a dupe. Lovely green in its own right, just not a Hunter Dunn stand-in.
The usual caveat: a deep green this saturated lives or dies by the finish and the light. North-facing rooms will pull it cooler and moodier; warm artificial light softens it. Whichever you choose, get a proper sample pot, paint two coats on lining paper, and move it round the room before committing — deep greens shift more than people expect.
And prime well if you're going over anything patchy. A coat of a tinted grey primer under a dark green saves you a third coat and gives you a richer, more even result.