Istanbul Card is known as Anonymous Card. The card is valid almost all forms of transport such as; metrobus, tram, subway and ferries. You can have this card paying only 6 Turkish Lira. And by loading money this card how much you want, you can take a ride everywhere in Istanbul.
The most ideal thing for family of two person is to take only one card. Because card can be used repeatedly. Therefore, without paying money from your pocket, you can go from hotel to museum or wherever you want by means of transport via this card. Anyway you can not use money for means of transportation.
For this reason; if you do not have Istanbul Card, you have to use single-use card in the metrobus or buses. But you may not find easily this card in everywhere. Because while you are waiting bus in any part of the Bosphorus, you may not find card so taking Istanbul card is more appropriate for you. You can buy this card all metrobus stops or central places like Eminonu, Taksim, Mecidiyekoy and you can easily fill up in these places. Also there are card filling counters and automats in every metrobus and tram stops. The occupancy of this card is two years. Reaching the automats is easier because they are found in every tram station. Point to consider for these automats you can not take remainder from automats. For this reason; you should leave the money how much you need.
You may not get back remainder and machine may not give it. Be careful on you load money from automats maximal 30 Turkish Lira so you do not have any problem. If you have extended family, you can raise this amount to 50 Turkish Lira. Finally you prefer the automat which has in front of the queue. Some machines can be out of order. So automat is tested.
For Detailed Information: http://istanbulkart.iett.gov.tr/en/istanbulkart/pages/menu/427