Spirit Games have limited codes listed, whether they are still in stock I don't know. (You'll need the codes though, if you are struggling I've got the original Navwar catalogue listings). Probably need to contact them direct to find out exactly what they have (on the plus side, they do list them at 75p or £1.10 a pack!)
https://www.spiritgames.co.uk/disp.php/5/12/11851/NaismithNavwar will be a dead end, unless a few packs fell down behind a cupboard - remaining stock was sold off with the moulds to Naismith-Roundway.
Alas, Naismith-Roundway has gone. I fear the moulds may never resurface. Am I correct in thinking that the 'new' owner has passed away?
Realistically, unless you just want a handful of packs you are probably going to be unlucky. eBay may well be your best source for a big batch of packs.
Elsewhere, other ranges often described as true 15mm:
Cavalry - Minifigs are a near perfect match with both Naismith and Heritage Napoleonics. I've got several hundred Heritage Napoleonettes, and a few thousand Naismith. There's quite a few Minifigs cavalry in the mix too. Minifigs foot are a little bit bigger than both Heritage and Naismith (at least a shako taller).
Essex are a whole head (& hat) taller.
Warrior have temporarily gone, but they have been unavailable for quite a few years now.
Magister Militum are an okay fit, slightly taller than Heritage and Naismith. Quite slight of body. Magister Militum are up for sale and not casting, just selling off remaining stock.
Minifigs foot are probably your closest best fit, that are still available - same army, but not the same base.
Sorry to be the bearer of bad news.
Update: Keep Wargaming are very much still in business (just placed an order, and had an email chat)