First of all, you need to go to the Slums of New Ark. I think you know where Don's lair is. If you go south from the gate of his house, you will immediately get to a place where there are many homeless people. One of them (he is standing to the left of the entrance, but not immediately, but a little down) can give you some valuable advice in exchange for seaweed or small fish.
In general, just poke around these homeless people - and you will quickly find the person you need. Unfortunately, he gives only one piece of advice per day. In order not to waste time walking, you can use the flophouse located in the same location (lower right corner, one night costs 5 credits).
The first advice will be about the bathhouse attendant Sammy, the second - about underground fights in the "Devil's Drink", the third and fourth - about the Forest Park and fairies, and the fifth time the homeless man will advise you to buy a fake pass to get into Upper New Ark. After receiving this advice, immediately go to the small market in the slums (up to Don's house, left, then down again). In the bottom row, in the middle, in one of the shops you can buy a "Bad Fake Pass" for 100 credits.
To turn a bad pass into a good one, you need to go to the masseuse Lin Lin and do any massage. In this case, a fake pass will fall out of your clothes, Lin Lin will notice this. For a small tip (20 credits) she will take you to the tattoo saloon, where you can turn a bad pass into a good one for 500 credits.