[Shovah32]

Seraphs. Nowhere near cost effective for an astral mage, but I had to say it.

Spectres(Death), Lamia Queens(Nature and Death iirc) and Tartarians(High Death) can all get astral from randoms. Tartarians are probably the best of the three and theres even a tartarian who only gets sorcery randoms, iirc its the female titan. Other than that all I can think of is Golems and various wished-for things.

There are also various national summons with astral magic, but it doesn't sound like you have access to them.

edit: Oh, and ether gate. 90 gems for, iirc 15 ether warriors(large, ethereal, weapons that kill magic creatures) and an Ether Lord. I think he has S2-3 and maybe D2. Or maybe its slightly random, not sure. Not exactly cost effective just for the mage(or even the whole spell), but its an option(S4D1 and 90 pearls).

edit again: Lamia Queens don't get astral, as Micah pointed out. And as Ylvali said, you can get lucky with Arch/Ice Devils.