What I've learnt: Great description below by Natalia.
I bought my 6TL Akbil from the blue kiosk at Taksim. It stands to the right of a red touristy looking one and is in the same area where buses are waiting to depart. It's not open all day and there is usually a queue so it's relatively easy to spot.
Refiling is simple: find a street shop selling drinks/candy etc where there is a blue Akbil sign. Hand over your key and state an amount to be charged. Even if you didn't say anything and handed over 10TL they'd work it out. They will give you a receipt showing the akbil has been charged.
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Teacher discounts are available to teachers working in schools or universities I believe (not private businesses).