Know it's been two years since the subject was posted but nevertheless I would like to offer an updated information on prepaid mobile internet / data plans in Serbia:
1. Best offering you can find at Telenor,
Telenor good 3G/HSDPA speeds, 2GB of traffic will cost you 1.290 RSD, that is 13 Euro.
2. MTS Telekom offer,
mt:s - Standard - Privatni korisnici is quite similar in terms of speed and cost
3. VIP,
Vip mobile | Po?etna, does not offer prepaid mobile internet?! even though they use MTS network infrastructure. Temporary perhaps.
SIMs and topups are available everywhere virtually, no question asked, does not matter if you're local or a foreigner. Prices in Serbia are generally lower than in surrounding countries (Croatia being the most expensive one).