It drop by mobs randomly or it can be purchase at junk shop with 2 different version (100K or 200K)
The difference between them is 100K version allow 2-8 people port into Dugeon. The 200K portal allow 1-16 people instead.
What's the point of Dugeon Portal?
The original idea is killing the boss to win 10K reward. It's ridiculous pay 100K to earn 10K, right? You damn right!
The main point of Dugeon Portal is XP. You can grab a such party in HQ and everyone purchase a Portal. Now that's equal 8 battles. The party set up need to be 2X shield + 6X AoE attacker (Mage would be the best). At Level 24, each battle should grant you at least 5% per battle, which around 40% XP increase after 8X. At the best you can try portal out from any level as long as someone let you tag alone upto Lvl28. At lvl28 only 1-2% per battle.
Tactics:
When you enter the portal, the 5 mins will count down, that's the max time you can stay in Dugeon. Follow your groups, 1 of the shield fighter will go down gather all mobs to one end of corner, that's the time when all mage can nuke with AoE. XP will be share but you get additional XP when you spells land the hit. Follow others to 2nd and others level to repeat same stage. Randomly the boss will be at any level, the 2nd shield fighter is responsible to hit the boss and pull it aside. If the boss die, you won the portal but that's not what you here for. Both shielded melee should be using 25+ equipment to be able to tank perfectly.
Warning:
The only skill you can increase in such battle is your AoE magic if you are mage or Shield/Defense if you are tank. The XP fly too fast for early Lvl20s so don't cry for your PT.