Your first concern in Crete will probably not be to buy shoes. Just as well because the choice is not that great and certainly not cheaper than what you will pay for where you come from. The exceptions is of course if you suddenly find that you haven't got the right type of shoes for what you want to do.
If you need sandals, there are two choices: plastic beach sandals which you will find in almost any shop near a beach and leather sandals which you will find in the "Leather Street" in Chania (and a few other shops).
You might also realize that the island is wonderful for walking
but that the rough, stony ground necessitates more than casual shoes or trainers.
You will find plenty of shops (in the cities) offering a good choice
of walking and hiking boots, not so much for the tourists but for
the locals who want to walk in good shoes as well when they are out
in the country.
Look for hunting supplies shops, there are plenty of them in Chania, or go to Karistianis (Skalidi Street) a long established shop for outdoors activities in Chania. They have a fair choice of walking shoes for every use and purse.
The prices are similar to what you would be paying in other countries.