Nixie tubes have been out of production for nearly 30 years. And thanks to the recent revival of Nixie clocks, the price of large tubes has gone through the roof. So if you want your project anywhere near reasonably priced and reliable, there are really only 2 tubes you can choose from, IN-12 or IN-14:
IN-14 is a side-view tube, while IN-12 is front-view. The digits inside are exactly the same, but IN-14 is more expensive thanks to its traditional tube shape. Still, those two mercury-enhanced USSR tubes have excellent reliability and easily last decades when properly driven.
exixe14 is compatible with IN-14 tubes, while exixe12 is for IN-12 tubes:
You can find those tubes on ebay by searching IN-12 Nixie or IN-14 Nixie. Most of the sellers are from Ukraine, and often you have a choice of NOS (new old stock) or used tubes.
NOS tubes are brand new and have never been used before. They are more expensive but they are the best bet when it comes to reliability and brightness.
Used tubes are desoldered from old instruments. You don't know how long they have been operating and in what condition. They have shorter pins than new tubes, often have cathode poisoning, sometimes even missing digits or pins.
For IN-14 tubes, I recommend buying new ones if you can afford it. If not, used ones work too but you do need to buy a few more than you need in case of duds.
IN-12 tubes are cheap enough that there are no reason to buy used ones at all. Its 2 variants IN-12A and IN-12B are exactly the same except IN-12B has a left decimal point. Both work with exixe12 modules.
There are a number of special tubes that has symbols inside them instead of numbers, usually for displaying the unit on a measuring equipment. Those tubes are cheaper than usual, but there aren't much you can do with them.
One example is the IN-19 Nixie tube. It is based on IN-14, so you can use it with exixe-14 modules, except all the numbers comes out as symbols instead. You can find a details about it on in-19_pin_mapping.md
Another example is IN-15 Nixie tube, which is based on IN-12, which is perfect for exixe-12 modules.
They are made of glass, so don't break them. They also have mercury vapor in them, so really don't break them.
Nixie tubes use a tiny amount of power and don't run hot at all, so it's alright to touch the glass while they are running. You can use rubbing alcohol and paper towel to clean the glass, although the alcohol might take the markings off.