I have to power an itx motherboard without the typical power supply option.
I found this: http://www.ebay.com/itm/PicoPSU-150-XT-150W-12Volt-ATX-Power-Supply-Cyncronix/160429089631?_trksid=p2054897.c100204.m3164&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20140407115239%26meid%3D8873486385218778677%26pid%3D100204%26prg%3D20140407115239%26rk%3D2%26rkt%3D21%26sd%3D110888383297
Does anyone know how that works exactly? What kind of power supply do i need to plug up to that? Just a standard 12v wall plug ac/dc converter? Or do I need a specific mA/wattage?
Depends on the power you intend to put through it. Further down their page, you can see they do a bundle with the adapters (like the laptop adapters). Basically 12v times amperage = higher wattage.
If you plan to run the PicoPSU to its limits (150Watts) you shall need a higher amperage device. (12v 12.5Amp)
It depends on how much power you require really.
the full 150 just to be safe
What that adapter will let you do is use a more standard single source 12-volt supply to power your motherboard and possibly a peripheral or 2 like a hard drive.
This is an excellent solution for anyone who builds a HTPC using a ITX mobo (because ITX is about as power efficient as it gets) and wants a fan-less power supply to keep things quiet. But it's not a very good option if you have larger power requirements like if you're stuffing a AMD 4+core CPU into your motherboard and trying to do some bitcoin mining that requires some heavy math calculations (which causes said CPU to suck down the juice). And that's because a typical 12V "wall wart" is not as likely to be able to provide anywhere near the same current even if we're talking about a laptop power brick.(The thing to remember here is that It's all about Ohms law.)
BTW, I simply used AMD as an example of a power hungry processor (which often requires more "amps" and thus a beefier PSU) since Intel has AMD beat pretty bad when it comes to power efficiency. But make no mistake, even a Core i7 or something (or even an older spinning HDD) might require a bit more "amps" too/instead (it depends on your choice of components in your rig).