Sue, I went to see Baris at Avea in Fethiye and he sorted a mini sim out for me for my ipad. It was enough for the two weeks use for emails/browsing etc and cost I think about 70 lira initially then you pay for top ups. It had to be registered and although you are supposed to be able to top up on line, I can't figure it out so when we return, I just go back and Baris does it for me for about 10 lira.
Sorry to be a bit vague but I haven't a clue on the detail - it works and that's all I know!
You do have to take your passport in for registering as you get a turkish mobile number too as part of the deal.