This is really several questions in several realms, almost none of them about electronics.
The first part is Metrology, the study of measurements, and in this case accuracy. Given that the LM35 itself is fairly cheap (roughly $0.65 to $3.00 USD in 1k quantities) and only 0.5°C accuracy (at 25 degrees Celsius) and +/- 1 degree C at the extremes of the scale, it doesn't make sense to spend too much money or time trying to calibrate or improve its accuracy.
The second is about the chemistry question of (safe) substances that have a boiling (or thawing) point below 90 degrees Celsius. The only substance used for standard platinum resistance thermometer (SPRT) calibration under ITS-90 is Gallium's melting point (29.7646 C) , and the triple point of pure water (0.010 C). I don't know about the toxicity of Gallium, but I assume that the lack of readily availability of Gallium negates that concern. Again calibrating it against anything other than pure water (deionized / distilled water) seems to not be worth the effort.
Distilled / deionized water is typically available in North American and western European grocery stores, combined into the section with bottled water. This isn't analytic laboratory grade, but decent enough for many home lab experiments. I have no clue on its availability globally, but if bottled water is readily available there is a good chance that it is available as well.
I would also be careful not to assume that any difference is constant or even linear over the entire temperature range. For example SPRT can be calibrated to 5 reference points based upon chemical properties of various elements over their range to help determine the calibration curve.
I would agree with Martin, that the comparison with a thermocouple is likely the best cost-effective and/or affordable approach, my only other suggestion is to compare the LM35 sensor to a known-good calibrated thermometer that is preferably an order of magnitude better than the sensor (i.e. +/- 0.1 degrees at the extremes of the scale, and +/- 0.05 degrees C at 25 C).
Because of Combined gas law, you also need to factor in barometric pressure as well if referencing to a physical phenomena (e.g. melting, boiling), rather than a relative comparison of two instruments present at the same time/place and thus same atmospheric pressure. Depending on your location, the adjustment may be small or large, depending on your location's elevation and the current local atmospheric pressure and temperature compared to compared to the standard conditions for temperature and pressure.
My apologies for what likely seems like overkill, but I don't want you (and others) to be mislead into assuming high-quality calibration / accuracy is easy or quick to achieve.
Hopefully this helps to explain what the potential factors are so you can decide how you wish to solve the problem in your case. Good luck.