I need to replace a small (@ 44 millimeter or 1.75 inch) fan in my computer, but I can't find the fan on the internet. It's made by Aopen (yes, I tried their website). For some reason, it says "sleeve bearing" on the fan. (I don't know why, it's a fan, not a sleeve bearing.) Of course, it runs on 12 DC volts.

Does anyone know how I can find this fan?

You should be able to use any similar sized fan that has the right type of connector; nearly any computer shop should have one. It doesn't matter if it's not an AOpen branded fan.

The "sleeve bearing" just means that it doesn't use ball bearings. The sleeve bearing is simply the fan blade slipped over the motor housing, like a sleeve. These types of bearings tend to wear out quicker than ball bearings but are cheaper.

If you don't want to worry about replacing the fan again, see if you can find a ball bearing fan.