Actually, using water might work. I'm thinking that some pseudo-distilled water together with air pressure readings from either my weather station or the local meteo station should give me a boiling temperature accurate to maybe within 0.2°C. According to the Javascript calculator, 5mbar of pressure difference cause a 0.15K difference in boiling temperature, and according to
this guy's experiment, one spoon of salt causes a 1.5K difference in boiling temperature. I can probably stay two orders of magnitude below one spoon.
I'm making a few assumptions here, namely that salts are the most common thing affecting the purity of my water, that all salts are in a similar order of magnitude as far as affecting the boiling point is concerned, and that a worn but clean pot won't add anything noticeable to the water. hopefully I haven't missed too much. I'm thinking that I might need to figure out some way to wrap the wires leading to the NTC, in order to keep them dry. Even though distilled water is supposed to be non-conductive, I doubt the stuff you can buy is really all that pure.
Another accurate temperature point I can get from a clinical thermometer. Those seem to be fairly accurate, even if their range is very limited. But I'll test a few first to see what the variation is.
As for the freezing point of water, that seems to be affected a lot more than the boiling point by impurifications. I could try saturating the water with salt, which should give me a fixed well know point.... except that all the salt I can get here contains Iodine, which probably affects the results as well. Oh. And it just occurred that satured salt water freezes at -21.1°C, and my freezer doesn't actually go that low. Doh.