Simple answer - you can't.
More complicated answer - there is a certain type of pack in which you can select four different clans - if you specify Jungo and three other clans, then chances are that you'll get a Jungo card. However, there is a caveat... if memory serves me, you only get three cards in the pack, so there is still a chance that you won't get any Jungos.
Currently, it's impossible to buy a pack that contains only Jungo characters. The pack that gives you the highest chance to get Jungo only characters would be Elite pack (highly improbable though), unless the staff release nothing but jungo for a few months in which case, the New Blood would give you only Jungo then.
You cannot pick what -one/specific- clan you want to get in the pack.
in least expense (20 creds) you can have the classic pack which lets you choose 6 clans.
so each of the three cards you obtain has only 1/6 chance of being in jungo