40 hole 26" rims
Have you tried SJS Cycles? They built the bike and have the widest stock of components around.
I have a spare pair of 26" wheels, 48 spoke Rhyno Lite rims on Shimano HF08 tandem hubs (I think they were J D tandems last new pair). They were built by J D for a Thorn tandem which we owned at the time but sold shortly after, so they have done just a couple of hundred miles and are in superb condition. We now have Rohloff so these are spare but I haven't got around to advertising them.
I would want £250 for these, but appreciate they may be overkill depending on your intended use. They are an immensely strong pair of wheels that were built for loaded touring / off road use.
If you have doubts about the condition of the wheels on the tandem you are considering it would be well worth checking the rest of it thoroughly too, components on a tandem have a hard life and there is lots of scope for other bits to wear out expensively too, sadly.
Personally I would stay away from 26" wheels. The industry is moving away from them and as you are discovering there is less stuff available. 26" is fine but not if its hard to buy replacements
If its cheap / good enough condition to use then fine with the knowledge that replacement stuff may get more difficult. Otherwise I personally would only buy a bike/tandem that is 27.5 or 29" wheel.
Well . . . I wouldn't dismiss all Thorns out of hand. They are excellent tandems, and don't all have 40 hole rims by a long stretch.
Bear in mind that maintenance of wheel hubs often gets neglected, so if the rims are shot then there's a fair chance that the hubs could also be past their best and benefit from replacement too in any case.
I'm sure SJS can probably supply anything needed to keep an older Thorn tandem going, just not necessarily exactly the same spec as the original component.
Thorn/SJS changed my 40 hole hubs to 48 hole ones when they did the most recent wheel rebuild for my vintage triplet. But the most trouble-free back tandem wheel I’ve had is a 32 spoke one with a Rohloff hub (also rebuilt by SJS for my Pino, after car crash damage to the original). Because this is not dished it is inherently stronger. If you don’t yet need new rims for the tandem you’re thinking of buying, it might be worth saving up for a Rohloff when the time comes. I would never now choose a tandem with derailleur gears – with a Rohloff it is so much easier to restart after a unplanned sudden stop. It might be worth checking with Thorn whether the tandem you are considering is retrofittable.
I've contacted SJS, and no, they don't 'do' 40 hole riims anymore. Hubs are145mm apparently, so swapping 36 spoke MTB wheels is a no (for the rear anyway), maybe I'd have to swap the axle?
What are those Sun rhyno rims like for getting tyres on/off, the CR one I had a while back were torture.
Bit of an update, i've asked the seller to check rims with a credit card or straight edge, and photo them.
I've manage to locate a small number of 26" sputnik 40 hole rims....YeSSSSSSSSSSSSS!! £25 each. I'll buy some, whether the bike needs them or not, this means I get to keep the schmidt dynohub (if it works haha).. Roll on Saturday...Failing that it was looking like a Dawes kickback twin , built up by SJS, but not in the same league me thinks., Here's hopin'