Beeswax for leaky canteen?

If you don't have a beekeeper in your area (try a local farmer's market looking for honey and work from there), I have gotten blocks of beeswax at Hobby Lobby. Look in the section of candle making supplies, the little bars sold where they keep leather craft supplies are too small.

Tough to say how much you will need. I would do it the same way I did my gourd canteen. Melt wax, in a double boiler, pour some in the canteen then slosh and roll, repeat as needed, pour off excess.
If the canteen already has an old wax lining I wouldn't heat the canteen, if it doesn't then maybe gently warm it before the 1st application of wax. After the 1st application don't rewarm it.