I don't know of anyone except Cretans who actually own an apartment or property in Agios Nikolaos itself which is available for renting as in my opinion, it's not a popular area other than for locals.
It's noisy, full of traffic even in the quieter winter months and the apartment sizes for the monthly rental that is sought are on the small side to say the least.
You may find something amongst this agent's details, which is the best I've come across for the area.
Or you may find something in the local paper which you could ask a Greek speaker to translate for you.
http://www.propertyincrete.co.uk/proper ... -1252.html
Happy house hunting.