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[Celtkhan]

I did a quick search, but all I managed to find was ads for people selling helmet fan kits. Since I’m not terrible at soldering, I figured I’d put together my own helmet fans off Amazon, but I need a bit of advice.

1. I already have a 5v USB power pack; should I get a pair of 5v fans, or are the 12s/24s more reliable?

2. What dimension fans should I get? I figure 50mm is probably about the biggest diameter I should go with, but would I be better off sizing downward?

3. Is there a particular brand that is both cost-effective and reliable? I’d like to keep costs down at this point.

Thanks!

[Fi-8015]

Take the 5 V Fans, the others would turn only slowly, if at all.
50 mm seems to be a standard size, they are available cheap. If you decide for smaller ones, you have to expect either less flow or more noise at the same flow rate. The opposite applies to bigger fans.
You should also choose the right kind of fans for your planned application. If you want to supply fresh air from the outside through an opening you can use both axial or radial fans. If you want to add circulation inside the helmet and don't want to cut an opening in your helmet you should go with radial fans (or squirrel cage fans).
I can't recommend a specific brand, but any should work fine. The only real differences  should be the power of the motor and the bearings, which can affect the noise level.