In Slovenia you can buy prepaid packs at mobile operator shops (Mobitel:
http://www.telekom.si/info-in-obvest...ekomovi-centri, Simobil:
http://www.simobil.si/poslovni/pomoc...prodajna-mesta). But also at most petrol stations, post offices and
newspaper shops. Normally no registration is necessary.
On the following page you can see some details about the prepaid tariffs:
http://www.mobilna-telefonija.com/mo...or/paketi.html
In the table "SI Mobilni paketi" select "predplačniški" for prepaid. Then in the last column one of the 14 prepaid tariffs available there.
For mobile data you might conssider one of the following two offers:
http://www.telekom.si/zasebni-uporab...bilni-internet
http://www.simobil.si/naprave-za-mob...icro-sim-paket