self-catering one room accommodation for visitors (Istanbul)

I am trying to help out a friend from South Africa soon to visit Istanbul who wrote:
My partner and me want to spend a month in Istanbul this year some time just to walk and experience the city I have been looking at the english papers etc to find the best and least expensive options for renting a self catering one room possie for a month but am not quite sure what the best options are - do you perhaps have any suggestions, advice or contacts for finding a relatively secure, inexpensive place from where we can explore Istanbul?