So about the gaining casting through Polymorph thing, what forms are there that have spellcasting of their own (so not SLA's, actual spells)?
There's those 2 Hobgoblin subspecies in MM5, as wel as the Kuo-Toa one...
Ethergaunts in Fiend Folio (Aberrations though, so would probably require you to be an Elan...)
EDIT: Found the Sylph and the Spell Weaver in MM2.
What more is out there?
Dragons, though they have a pretty bad HD/casting ratio, maybe with the exception of the Tome Dragon, from Dragon magazine. I don't believe I have that issue, so I'm not sure of its specifics.
A lot of good outsiders seem to have cleric casting.
Here's what I found (in the
SRD, BoED, and MM2):
Outsider: Planetar (cleric 17), Solar (cleric 20), Trumpet Archon (cleric 14), Firre Eladrin (boed, cleric 12), Ursinal Guardinal (boed, wizard 12), Lillend (bard 6), Sylph (mm2, sorcerer HD+4), Arcanaloth (mm2, sorcerer 12), Couatl (sorcerer 9), Rakshasa (sorcerer 7), Lammasu (cleric 7 plus two domains of good/healing/knowledge/law), Formian Queen (sorcerer 17),
Magical Beast: Aranea (sorc 3), Androsphinx (cleric 6)
Monstrous Humanoid: Abeil Queen (mm2, druid 16), Spell Weaver (mm2, sorcerer HD+2)
Elemental: Air Weird (mm2, sorcerer 18 + domains air/travel as sorc spells), Earth Weird (as Air Weird except earth/destruction domains), Fire Weird (as Air except fire/sun), Water Weird (as Air except water/healing), Immoth (mm2, sorcerer 12 + can't cast fire spells),
Dragon: Gray Linnorm (mm2, cleric 17, destruction/evil domains), Dread Linnorm (mm2, sorcerer 18)
Fey: Nymph (druid 7)