Evil eye is a classic gift. It is believed to be a shield for the negative energy coming from peers. You should hang it at the entrance of your home. Thus, it should be the first thing that people see.
http://perceptivetravel.com/blog/2010/09/07/the-evil-eye-in-istanbul/
Other than the evil eye, you should look for leather products, ceramics, and hammam supplies. Leather products can be found at the various bazaars, in the Aksaray area, and if you are willing to go for a long metro ride, in Zeytinburnu. You can get great stuff for a fraction of the price that you'd find in the states, but expect to bargain.
Ceramics can be found pretty much everywhere. I'd avoid the tacky calligraphy (unless your mom is an Ottoman history buff) but there are a lot of really interesting floral motifs or small figures. Look for bowls, which can be mounted on the wall as a decoration, or little mugs and things.
Finally, if you go down by the galata tower, you can find a ton of stores selling upscale bath supplies (also check the bazaars). A turkish bathrobe or scrubrush is always a nice gift, but I especially like the thin plaid towels they wrap around your waist in the hammams.
Also, if your mom is like mine, consider wine? I like Angora and Karga, both of which are less than 20 TL and feature turkish varieties of grapes.
