From Gotham's location and geopolitical status, moonsugar, the essential ingredient to brew skooma, can only come from khajiit nomadic merchants. Gotham's pollution levels are too high for moonsugar to be collected from the moon. As we all know, true moonsugar comes from the moon and it falls from the sky at night. It's what some call eyepoop or eyesand when you wake up. The other type of moonsugar are substitutes, like opium, cheaper, more addictive and poorer in quality. Chief polluter is Batman, the Dragonborn, with his stupid lights and dragons in the sky.

Now, while khajiit merchants are known to be thieves and criminals, most are not. This is racist propaganda perpetuated by the nords and their longtime ennemies, the woodelves. It is also important to understand that quality moonsugar and skooma is part of khajiit culture. They hate the, more addicting, bad stuff however. As such, even if all types of skooma and moonsugar are illegal in Gotham, punishable by a batpunch to the face, khajiit merchants will still bring some with them but will only trade with other khajiits.

Now while the cheaper stuff, like Mr. Freeze, is sold by many other, smaller, criminal gangs in Gotham, the only criminal that could obtain the moonsugar necessary for the brewing of high quality skooma from the nomadic khajiits is Catwoman, dubbed as such by CEO, professional at looking like an asshole and anti-khajiit activist Bruce Wayne.

However, it is suspected by the authorities of the city that the only criminal capable of brewing the skooma currectly is Poison Ivy.

I hope that answered your question. Remember, while this happened in the past, it's also happening now and in the future due to time paradoxes caused by new technologies and the Elder Scrolls.

http://giphy.com/gifs/the-simpsons-eyes-shifty-ebuZ7sDhZcSJO she did do this once and that's really all the evidence that's needed