There are several ways to grind sandspitters.
If you haven't finished the Avikan mech guide quest yet, you can simply start the quest -- you'll find exactly two sandspitters in that instance, and right after killing the second one by just teleport out. Then you can enter the instance once again for two more sandspitters, and so on.

If you already finished the quest, you need to find the planet with a wetlands biome instead. It should be easier it if you have "More Planet Info: Biome Fix" mod.
1. If the surface biome is large enough, but there were not enough sandspitters in the first run (or none at all), there's no need to look for another biome -- just keep running from left side of the wetlands biome to right and back as many times as you need to -- the sandspitters should keep spawning occasionally, and you will likely have anough kills and materials within half an hour.
2. Or you can try digging underground and search for underground wetlands biomes. They usually spawn under the surface wetlands, but sometimes they spawn under other desert biomes as well.
In both cases look for large pools of quicksands, since the sandspitters usually spawn somewhere nearby.

Hunting the other Elithian creatures (farynths for example) is pretty much the same -- look for a good place for creatures to spawn, then just run back and forth untill you kill enough. Don't leave the planet, don't run around the whole planet -- just around the suitable area.