Another suggestion is to find creatures that carry the reagents, that you can kill more cheaply than the reagents used to kill them.
For example, necromancy reagents come from liches, fairly easily (and you could end up selling them to a fellow Sieger that's a Necromancer for money or trade them for mage regs). Corpsers, reapers, and the spellcasting ghost-like undead (spectre, wraith, shade) have magery regs.
Moonglow has 4 or 5 different spots (if you count Papua, which you get to through one of the Moonglow mage shops) to buy regs from, and each place has two or more reagent sellers.
Another idea, if you've got 50 magery already, is add enough cartography to do level 1 treasure maps. 50 Magery will unlock the chests, last I remember, but without detect hidden and remove trap, you'd have to take damage by manually tripping the chest trap by opening it up (telekinesis doesn't work on treasure chests on Siege). If Unlock doesn't work on the chest, you can always go and train lockpicking to 33 or so.
Each level one Treasure chest has 3 stacks of 40-60 regs, and you can do 12-20 of them an hour, if you have access to a rune library (I'm sure one of the SP guilds can point you to a map rune library) and ability to Gate around. If you have to cast Gate by scroll, remember to change the target location to default, and target it with the scroll.