My choices:
1. Ratmen cave at Sacrifice.
2. Giant Serpents in the cavern between Level 1 Destard and the ramp down to level 2 (you can get bones and horned leather there as well, and the dragons rarely wander in there).
3. Valor, Amoeba and Bald islands for Giant Serpents and alligators.
4. first level Rikktor, Barracoon or Semidar (imps give spined too) spawns in Ilshenar (use butcher's war cleaver to cut leather directly into your backpack, and BoS it back when full - spawn will keep you hopping). Also, 3rd stage Oaks Spawn (Centaurs). The normal centaur spawn north of Mistas is good as well, and either centaur spawn will allow you to farm arrows at the same time.
5. One of the slith types gives spined, and I think at least one boura type does as well.