Start by finding the original make and model number(s) of the fans you want to replace. Use that info to search Google for the specs of these fans. To be sure it will fit in your unit, in addition to the diameter (80 mm), you want to know other mounting dimensions including the height and the location of the mounting holes and access to them.
Here are pics of fans with conventional and non-standard mounting:
Then, you'll need to know:
1. The operating voltage and current requirements and whether it runs on DC or AC.
2. The airflow -- Usually given in cubic ft/min or (cfm).
3. The rotation speed -- Not as important as the airflow, which is related, but a slower rotation may be quieter.
4. The type of bearing -- Fans that use ball bearing tend to last longer and be of higher quality than those using sleeve type bearings, and double ball bearing units are better than single ball bearing designs. As you would expect, ball bearing units tend to cost more. However, if the original fans in your cooler use sleeve bearings, you can expect similar performance from other fans with the same type of bearing.
Of course, even between fans using the same type of bearings, some will be of better quality than others so try to check the reputation of the manufacturer.
5. The number of wires going to the fan (usually two or three) and the type of connector. Typically, fans with three wires are speed controlled. If you find the right fan with either the wrong connector or no connector, you will have to be able to use the connector from your old fan or find another new one.
Where you buy them depends as much on where you are. If you were in the U.S., I know of several outlet stores that buy closeout quantities of high quality electronic products and resell them at relatively low prices. I know you are not in the U.S. so availability and means of payment and shipping from those vendors are issues. You will have to determine your best sources for the fans you find that meet your requirements.