They should specify what connector it uses. Either 3 pin or 4 pin(molex) If you get a pretty generic one, they are typically going to have the Molex connection. That'd work just fine, but it'd be powered straight from your power supply and not the motherboard. The only difference is your mobo couldn't control the speeds or give errors if it's going to slow.
Also, if this is a replacement for your CPU fan, some motherboards do require a 3 pin fan just so it can alert you when the fan's going too slow or even not connected.