Don't know about other sources, but if you get the STA-Travel Blue Ticket, it is valid for 12 months and changes are $25. However, this ticket is only offered by a few airlines, I know of BA and Qantas.

A STA-Travel agent told me, that there are seperate contracts between airlines and the local, i.e. country-specific, STA agency.

For tickets I bought in Germany I had the following rules:
[carrier, route flown, booking class, student discount, penalty]

LH, Germany - US, any (I believe you can pick any fare V or higher for 12months stay), 10% off regular fare + open return, EUR100 changes after that

UA, Germany - US, W, special fare + 1 change of return flight free, EUR100 after that

BA, Germany - US, ??(one of those that earn 25%), regular fare but any change (routing and/or date) $25

all of these tickets had a maximum stay of 12 months.

Oh, and one important restriction: Usually you can only get student fares originating in the country you are buying the ticket.

Feel free to add to this, maybe we can have some kind of collection of all available student fares/websites with pro and cons...

David

Disclaimer: I don't work/am affiliated with STA-Travel, they are just the most conveniant TA in Germany for me...