I was in Istanbul just a couple of days ago and I rented this mobile wifi from a campany called
Alldaywifi.com
They brought the mobile wifi to my hotel in Old City and I left it in hotel's reception in Cappadocia.
The speed and the coverage were good, prices are cheap enough and they reply about small issues immediately.
I also bought Istanbul Card and 3 sets of Fast Museum Cards. I highly recommend those because Istanbul Card provides discounted rates on transportation and the museum passes saved me from long queues of the museums.
I noticed that they started to offer airport delivery and airport transfer but at the time I booked, they didn't offer these services.
They met my basic tourist needs perfectly
I think they deserve to take a look