Here are some popular options for English language courses in Istanbul:

1. British Council: Offers high-quality English courses, including general English, IELTS preparation, and business English. Their courses are recognized worldwide, and they use experienced native and non-native teachers.
2. English Time Language Schools: A well-established English school in Istanbul, offering a variety of courses ranging from beginner to advanced levels. They have multiple branches across the city.
3. Berlitz Language Center: Known for its immersive language learning method, Berlitz provides English courses for individuals and professionals, focusing on communication skills.
4. Dilko English: A popular chain of language schools in Istanbul, offering a variety of English courses including exam preparation and conversational English.
5. Wall Street English: Offers personalized and flexible English courses, catering to different levels and goals, with a focus on practical English for everyday use.

These institutions offer flexible schedules, so it’s best to check their websites or contact them for specific course details and timings.