eBook referencing, How to differentiate "Available throught" and "avalable at" ?

Good day,

My question regards electronic Books and Chapters. Some references should use the “available through”, if they are password protected items, some others use “available at”. It is easy to setup a reference style using one of these, how can I set up the style to use one or the other, without creating new reference types, which would be quiet a lot considering two different types for electronic books and two different types for electronic book chapters (edited books).