You might try huntington's (www.huntingtons.com). They show a box of 20 Norma cases for about $45 USD. However, with Norwegian import taxes, shipping, etc. you will probably be back to $5 per case. www.midway.no offers 100 cases for 3203 kr which, as you say works out to almost USD 5 per case.

A further option might be one of the UK auction houses, especially Holts. Sometimes they offer brass in their 'sealed bids' sales, where you can get some occasional bargains.