Most of the bigger electrical shops, and some motorcycle shops have generators for sale.
What size you need depends entirely on how much power you want from it. Of course, the more powerful they are, the more expensive they become. For a generator capable of running fridges, TV, cooker and lighting during a power cut, you've got to budget from 2,000 YTL upwards.
You'll also need to consider where you're going to house it, how you're going to fuel it and where you're going to store the spare fuel.