(Spoilers, duh!)
Go to the part of the slums where all the hobos are, one of them will mention a junk seller who sells counterfeit passes. These only have a small chance of working tho.
When you’ve visited Lyn-Lyn a couple of times and tipped her well, she’ll tell you about a place that fixes them, giving you guaranteed access to UNA tho it’s quite expensive.
Passes are a one-time use, so once you use it it’s gone and you need to get a new one. You can however get a repaired counterfeit pass and repair others yourself using an ink pen, but there is a good chance that you’ll ruin the pass.