Serbia

Availibility
You can buy them in every mt:s store. You need a passport to register it. Address is not needed.

Data packages
See http://www.mts.telekom.rs/index.php/data-plus-prepaid-dopuna.html. They can be activated on any prepaid, even though this might not be clear from their website, it works in practice - just send the SMS as explained there. The packages are valid for 60 days.
 * 250MB for 250din
 * 1000MB for 700din
 * 2000MB for 1200din

Recharging
There is no  possible way to do it from abroad. Trust me, I have tried everything, you need a Serbian credit card or you need to do it in Serbia. They don't use scratch cards any more, you need to tell the vendor your numer and it gets recharged electronically. Each recharge (no matter the amount) extends the validity of your card for 1 year.