I did a little googling and I think it is not practical trying to convert AC motor to DC motor. Fan is nothing more than a motor with blades on it.
I guess it will be more easier (economically and time-consumption) to buy DC motor. Just attach fan-blades structure to it and voila, you have a fan. Place it on desk and you'll have desk-fan or pedestal fan. Hung it from ceiling and you'll have ceiling fan. Just keep the structure simple and crude. No need to do fancy finish.
To learn more about fan-blade-structure then if you have access to "Lahori-style cooler" and pay attention how they put fan-blades structure on just an ordinary AC motor. Put same structure on a DC motor and it will be DC fan.
As DC motors usually comes with permanent magnets, that's why they are bit more expensive compared to AC motors. To save some bucks, try to get some used DC motors. Maybe used DC motor from a treadmill. Some treadmill have AC motors but "quieter" versions usually comes with DC motors.
Or if you are persistent enough then you can make your own DC motor. It's not that difficult, if you have spare times and you can get your hands on good quality magnets. I guess you can find tutorials on how to make your own DC motor on YouTube or Google. Good luck and keep us posted on the progress of your project.