The instructions for the Parkside Models Grampus ballast wagon kit state that the model should be detailed with "ModelMaster 2631", available from the N Gauge Society.
The N Gauge Society shop shows none in stock (https://www.ngsjoin.com/br-1948-68-freight-stock-decals-5143-p.asp), and concerningly, another part of the website (https://ngaugesociety.org/modelmaster/) discusses ModelMaster in the past tense, suggesting that they no longer offer their products.
The ModelMaster website itself offers none of the relevant decals for N Gauge - they only offer Grampus decals in OO Gauge.
Are these decals still produced and available by the NGS, and just temporarily out of stock, or am I now out of luck with a kit whose instructions are dated 1998?
They're in the process of being sorted and made available again. I'm sure I've read a more recent update than this but it's the most recent I can find on here. https://www.ngaugeforum.co.uk/SMFN/index.php?topic=44666.msg597657#new (https://www.ngaugeforum.co.uk/SMFN/index.php?topic=44666.msg597657#new)
You could be in for a long wait. I was looking for some three years ago.
Just take a look at the Cambridge Custom Transfers who do a whole selection of 2mm transfers for these wagons depending on region etc.,