I'm going to be in Istanbul in 3 weeks for the first time. can you tell me where I could buy some please
Just don't get the boxed stuff, and get a lot of variety of flavors. Also I seem to be the only person who hates the rosewater, but maybe that's just me.
For baklava, it's really a labor of love and there's a lot of great old and small shops. Gulluoglu used to be great and is still quite OK, but feels a but mass produced now. Smaller options are Köşkeroğlu, kafadaroğlu etc, but I don't know if it's worth seeking out especially - just duck into whatever small shop that seems busy during the day.
Probably what might be a better experience is trying /unfamiliar/ desserts: Kunefe is crispy shredded puff pastry with cheese in it, doused in syrup, served HOT with pistachio on top. Tulumba is the turkish take on churros. Halva is made of tahini and sugar, and is getting more popular in the US. Kazandibi is pudding that's been caramelized / burnt at the "bottom of the pan" - traditionally the gelatin in it comes from chicken breast (you can't taste it). More often it's now the "fake" version, still delicious. Lokma is turkish fried dough. Kabak tatlisi is made of pumpkin. There's so much to try, just get the stuff which you don't know what it is.
One thing to remember is that Turkish sweets are quite sweet - you're only meant to have one small piece. Of course that won't stop many of us from getting a whole plate :-) Usually you can ask for a half portion at a shop.
Finally if you're near Kadikoy, hit up Ciya too - regional foods from across Turkey that you can't find in many other places.
Hope you enjoy your trip!
Also you might also like tahin pekmez - which I haven't seen around at all yet: 3/4 tahini and 1/4 molasses mixed together for a spread - on toasted bread. Best warming winter / snowy weather comfort food ever! :-)