Try Solenttools.co.uk

I am fairly certain that this distributor of Far Eastern products went out of business at Doncaster years ago but it was only a Nu-Tool 'badge' on generic machines which other similar firms sold. I would look at Machine Mart, Axminster et. al. to see if they list, or ever listed, an identical machine in their catalogues but I have a feeling it may be cheaper to simply replace the motor rather than bother with a new armature even if you can find one.

As Chris said, NuTool disappeared from the UK many years ago, though part of the company does seem to survive today in Poland

Companies House show the company dissolved in 2012, having last posted accounts at the end of 2010.

Their large Doncaster distribution centre was visible from the A1 southbound. IIRC their demise came about due to their being the supplier to B&Q of that store's "Performance Power" branded power tools – or rather an over-reliance on that customer & an inability to survive when B & Q moved to another supplier for the products they had supplied.

I have a NuTool branded 6" x 4" bandsaw which, apart from the makers name sticker, doesn't appear to be any different to other branded versions of the generic Far Eastern 6" x 4" bandsaws. If you can find another brande version of your belt sander from a supplier still trading yoiu may strike lucky. But the reality is that a NuTool branded items will most likely be 20 years old (or older ) & the chances of finding a spare part like an armature will be slim.

Least expensive option would most likely be to replace the tool with one from the middle isles of Aldi or Lidl when they have the item you want on offer – I have seen portable belt sanders there in the past. Power tools are pretty much disposable items these days.

Nigel B