For the details: Olivia can cross dimensions with her miiiind, and she needs the drugs to help give her that extra push out of the city. It won't work obviously, but she doesn't know that for sure yet!
However.... money is the one issue (which is, of course, a big issue for Yusuf). Olivia doesn't have that much on her since she just arrived. How much would he charge for a simple mix of psychotropic drugs, including LSD and other things that Fringe doesn't specify. If it's a few hundred (or whatever that equivalent is in city coins), I'm sure she can find a way to get it.
This FBI agent could also offer a favor in exchange? An 'I owe you one, if I don't leave' kind of deal? sob. She would also be willing to let Yusuf watch the entire process if he'd be interested in dimension crossing.