You should be looking in Google for:
simobil simpl sim "cena"
or
simobil simpl sim "price"
Top-up vouchers are called "SIMPL vrednostna kartica".
Ask for the price for delivery to your place. English should be understood by all shops.
Here is a link to one of the shops which seems to deliver to EU countries as well and without VAT if you give them your VAT number:
http://spyshop.shopamine.si/en/dodat...artica-simobil