The best thing would be to somehow gain the ability to summon undead capable of creating spawn, which you could then control. If only that existed. Oh, wait, master of shrouds. Hell, you could even just use Summon Undead V to summon shadows, which then create spawn before they disappear. You then command the spawn, have them create more spawn out of animals that you pull out of the dozens of grey bags of tricks that you have lying around because those things are dirt cheap, then have all the incorporeal undead jump into your bag of holding, and reach in there when you need some metamagic. Assuming ten bags of tricks(9k total), you're generating 10x10= 100 animals a week, and at 3 HD per shadow that's 300 metamagic levels per week. Not bad for 9k gp and a feat.