Welcome to Istanbul.
Here are some useful tips:
1-Don't buy any blades from Grand Bazaar. Come to think of it, don't buy anything from Grand Bazaar.
2-When you go to Topkapı Palace Museum, don't forget to buy the new Armoury catalog. It is quite good.
3-After visiting Palace Museum, go to The Istanbul Archeology Museum which is in the outer garden of the palace. You can easily walk there.
4-Visit Military Museum in Harbiye and Naval Museum in Beşiktaş.
5- Antique stores in the old city( Sultanahmet-Beyazıt, which isthe tourist center of Istanbul) are overpriced and full with fake arms and armour. If you want to go antique hunting, try Çukurcuma district(close to Taksim), Bahariye district(close to Kadıköy). I doubt you'll find any Turkish stuff except for19th-20th century Anatolian yataghans, qamas and knives; but you'll also find suprisingly large amount of 18th to19th century European swords, sabers, smallswords, bayonets etc. as well. And most antiquesalers have no idea what they are.
6-Keep in mind that every antique store clerk will try to rip you off, and mostly they don't know anything about armsand armour so don't be afraid to bargain. You might get lucky and find some great stuff for relatively cheap prices.
7-It's rather chilly and rainy right now, so take a jacket with you
Have a good time...