I try to keep a stock off Newtone Heritage 0.12 and 0.11 strings in my shop, but sometimes I only have enough for the ongoing batch. I also usually have a few NH 0.10 sets and regular Newtone Masterclass 0.11 sets in stock. Right now the stock is low, but a new load is on the way. It usually takes a couple of months from order to delivery, it seems that their website is often out of stock as well. Something happened during the pandemic and after Brexit, I hope it straightens out. With new prices for customs and shipping, they now cost SEK 140 per set if I have strings to sell. Postage is usually SEK 30 or SEK 45 within Sweden depending on the number of sets. If you want to order, send an e-mail and don't forget your name and address. I also have single strings in stock, it happens that one of the spun strings does not quite intonate as it should or that a thin E string breaks off when measuring the intonation.
I would also like to remind you that you should never cut round core strings during assembly before they are in place and tuned. The top 10 centimeters of the string is flattened during manufacture so that the string does not spin up, if you cut the string at the top first, there is a great risk that it will completely lose its intonation! I have also been told that the unspun strings in NH are no different than regular unspun strings, so they can be replaced with strings of the same thickness from other makes.
There is actually another similar low tension string to buy from Thomann in Germany, Optima Vintage flex. It seems to be more or less a copy of the NH 0.12 with the same string thicknesses and low tension. I have not tested them myself, but reviews online say they are equivalent to the NH strings.
Another low tension string with roughly the same tension as NH 0.12 is GHS Thin core Light.
Otherwise, it always works with any steel strings with normal tension if they are just a notch thinner than the NH strings. Then you get approximately the same string tension that the guitar is built for. Pretty much all GammelGura get NH 0.12 and there you can instead string up the guitar with normal 0.11 strings. If it's an unusually brittle old guitar, I use NH 0.11, and then you have to make do with a 0.10 set of normal strings. The intonation of all GammelGura are measured with NH 0.12 or 0.11 strings. The intonation will not be exactly as good with thinner normal strings than the NH strings used when doing the measurement, but the difference is small and hardly audible to a normal ear.