Googled them myself out of curiosity. They are still at the same address, but their telephone number has changed.

TÜRKMEN KİTABEVİ
Sahaflar Çarşısı No: 19
BEYAZIT
Tel: 011 90 212 512 22 71
(011 for international, 90 for Turkey and 212 for Beyazit, Istanbul.)

No website that I can see, but active in internet bookselling. Suggest you find your local Turkish consul and email him the photos – he/ she will help.
Interesting find, what does the gun look like?
Rs,
K.