You could put a 200mA 12V supply or a 200A 12V supply on one of those fans and they'd take exactly the same amount of power. However, I agree with your tendency to pick a supply with a current rating more appropriate for the load, as this minimises cost & space, and (assuming the supply is over-current protected, which may not be the case with some cheaper smaller ones) minimises damage in the case of a fault. If it's too small though, it will run hotter and less efficiently, and may present a risk in itself. I usually aim for a supply with 50-100% overhead on the load (much like we do using 30A supplies to run SD's), as this usually covers a wide range of quality and uncertainty about the true nature of the load.

Most DC fans have their current draw (0.1A, 100mA for example) written on the label. Add these together, and make sure whatever it's connected to is rated to 50-100% more than this.

A more conservative method would be to use fuses to protect separate circuits - e.g. have individual fuses for the heater, hot-end, steppers (i.e. board) and auxiliaries (e.g. fans) all run off one good quality high-current supply - much like a car, where batteries can deliver in excess of 200A if shorted. Panels for doing this are available from electronics shops; people use them in boats, caravans and excessive car audio setups . This would provide maximum protection from short-circuits and other faults, and probably represent something close to best practice. For these machines, it's probably a little on the extreme side of course, but in the light of recent events on this forum regarding fire risk maybe something we should all be thinking about a bit more.

[/constructive pedantry]

... Or should I just go the easy/safe way and just stick a 9v battery to the side of the case and wire them to it? I haven't actually tried, but I don't know if a 9v will power 2 12v fans for any length of time and fast enough they don't burn out. ...

Before you went with a battery, I'd just use a 12V AC adapter. Something with low mA, like ~200mA or so if you can find one. Thrift stores are a great place to pick up one for a buck or two.

A higher "Amp" rating probably wouldn't hurt it, but I lean towards not overpowering things like fans whenever possible.

Something like this:

The Y-axis stepper is heat-sinked to the case so it can wait until I get a power supply (probably a couple weeks) then I can run it straight from a 12v source.

Thanks for the tips!

When you put the extruder fan on, you'll likely want to run that off the g-code controlled fan output so you can have it turned on and off during the print.

Have fun!

I know the x/y stepper motors of the SD3 get really hot, so I got a few 40mm fans that arrived today and I'm wondering how best to wire them in. I know there is a fan over the extruder stepper motor (which I'm replacing because its bearings were DOA - it hurts the ears to listen to) which already has its own motherboard header. No problem there - snip snip, solder solder, shrink-tube shrink-tube, done.

Now, the next 2 fans for the steppers.

Can they solder to the same header? Can that header feed power to all 3 (maybe 4 in future if I get an extruder fan) 40mm fans? Should I splice off of the bed heater wire because it can handle lots of power? Or should I just go the easy/safe way and just stick a 9v battery to the side of the case and wire them to it? I haven't actually tried, but I don't know if a 9v will power 2 12v fans for any length of time and fast enough they don't burn out.

Any suggestions?

P.S. a new power supply with multiple outputs isn't in the near-future plans yet. Eventually I'll be getting a replacement PSU which I'll be able to tap into, but I want these fans installed ASAP to avoid killing my motors - they get way too hot for my comfort (I don't trust my temp gun that says they're 50 Celsius anyways).