I've hatched well with out a fan, with a incubator fan, with a computer fan, and in a card board box... ok well, I was like 14 with the box hiding in a closet so that didn't go so well...
If you get a computer fan that pushes enough air and get it hooked up right, it will work, but if you don't really want to mess with wiring... going with the kit will be eaiser to do.
I went the pc fan route on my 3 incubators. It cost me just under $10 for each 3 speed pc fan at Office Depot and Staples.
It took asking a couple people in the chat room to figure out how to wire a 12v dc pc fan in a 110v ac incubator.
I found by frying a fan that ac and dc do not work well together!!
After talking with people in chat, I found out to use a couple old cordless phone plug/cords. I used two 12v and one 9v adapter/cord/plug.
The pc fan has 3 wires, white, red, black, the cordless phone ac adapter has a negative and positive wire.
I stripped the wires about 1/2 inch, connected the red to the positive, black to the negative, using wire nuts, and left white alone.
I plug them all in and they worked.
So, with one hour of work, some on hand junk, and $30 worth of pc fans. I was able to turn 3 still air incubators into incubators with fans, for the price of one factory built fan.
My brother just put a PC fan in one of my bators, we used my charger cable for a flashlight to power it. It is a little nosier then the turbo fan, but it works good.
The PC fan works well if wired correctly. I have 4 60mm fans around the incubator and that thing pushes WAY more air flow than just once centered piece. In comparison in air flow, I think the four fans I have will push a WAY more efficient flow than the one that comes stock. My set up cost me $0. Go to PC stores, they sell those small fans for SOOO cheap...
Be careful to connect the wires right and be SAFE. False wireing will lead to fires!!!