These have not been carefully screened for size-availability or location relative to where you live, but here are a few sources you might check with. All report having larger-sized Araucaria araucana available.

Architectural Plants Ltd.
Nuthurst, Horsham, West Sussex, RH13 6LH
01403 891772
or
Woodgate, Chichester, West Sussex, PO20 3SU
01243 545008

Dulford Nurseries
Cullompton, Devon, EX15 2DG
01884 266361

Evergreen Conifer Centre
Rock, Near Kidderminster, Worcestershire, DY14 9RB
01299 266581

Mallet Court Nursery
Curry Mallet, Marshway, Taunton, TA3 6SY
01823 481493

Rare & Exotic Plants / Woodshoot Nurseries
Kings Bromley, Staffordshire, DE13 7HN
01543 472233

Thornhayes Nursery
St Andrews Wood, Dulford, Cullompton, Devon, EX15 2DF
01884 266746

Weasdale Nurseries
Newbiggin on Lune, Kirby Stephen, Cumbria, CA17 4LX
015396 23246

My first post - please make allowance for my ignorance. I have an easily-accessible 25 foot monkey puzzle tree, perhaps 30 years old, to offer in SW England. Can anyone advise their experience/success/logistics of transplanting such trees? All advice gratefully received. Thanks.

Your odds transplanting a tree if that size and age, of any age, are about 50-50, and probably not even that good.

This is nothing against you: homeowners are much too picky and impatient when it comes to planting yard trees. They want a tree that comes presized so they dont have to wait the four years for the tree to grow to the size they want. This is very frustrating for me. Smaller transplants have a much better initial success and much fewer long term health issues (SGRs and all that).

Ask yourself, are there any real negatives to starting small? It will be cheaper, and you have a better chance getting a small tree to survive the transplanting shock so you wont waste a couple hundred bucks buying a big tree that will die or a couple thousand teying to move a mature one.