AFAIK it only appears on desert, grass, plains, and tundra hills.
However, it shares grass & plains hills with coal, which can also be pretty common in GS.
By extension, playing Civ’s who have start biases for grass & plains hills (Greeks, Inca, etc) or mineable resources (Nubia) or even specifically iron (England) helps.
As others have have, niter can appear in the same terrain but on flat land only instead of iron being hills only. Adapting to no iron usually means rushing for crossbows and then military engineering to look for niter. If you have horses, horsemen -> coursers -> is a very good unit line that only needs horses for 4 eras.
Ultimately it’s all about oil anyways; you’re only missing out on swords, knights, and cuirassiers. There’s a lot of other units to use.