If you don't really want to roleplay it, this is what contacts are for. One of the players "knows a guy". Someone they met in a dive bar once said he could hook them up with cram and handed him a card. The dealer will demand a transaction and/or straight up sell you the contact info for a fixer. BAM, done.
But I think the idea of throwing new players into naive characters with no contacts in the shadows is a fantastic idea. It would most certainly force the players to think outside of the box and try and learn as go, just like their characters would be doing. It's a very true form of roleplaying. If you already ran it, I want a report.
What they can't do
If their legal lives were just destroyed and they have no contacts and no solid knowledge of the shadows, then life is going to be rough. Experience is valuable, lessons are expensive. If they have no SIN, and obviously no fake SIN, then they are barred from even the most basic of things like hailing a cab, getting a plane ticket, eating at a restaurant, spending funds, or withdrawing money to a credit stick.
Their legal lives might have had rights and money and friends. If that came crumbling down, the rights are the first to go. Being illegal means the system no longer works for you. Most of their savings would be seized or access restricted. The money they have on hand (or the things they can hock) could be vital. Most of their friends won't risk their cushy life, but some may be true friends.
Alternatively, even if their funds haven't been frozen yet, they don't have much they can buy with it. They can't buy anything illegal. They probably don't know enough to convert their money into funds that are beyond the legal system's grasp. No SIN means they can't even properly try and run from their problem. When they DO find someone that says they can hook them up, that person should almost definitely take them for a ride. Indeed, being the only point of contact for a group of desperate people willing to break the law for a buck can be highly profitable. The sooner they learn that criminals cannot be trusted the better off they will be.
I would steer them towards the barrens. The dumps. The low-class hive of villainy where their lack of a SIN won't summon lonestar, and people will take their money without asking too many questions. Throw a mugging at them. Some sort of bum trying to roll them for their shoes. They could try and simply ask around "hey, I need to hail a cab, and I don't have a SIN, help me out". This will draw out the con-men and set them up for abusive relationships. And the honest friendly helping hand will be indistinguishable from the one holding a knife.
What they can do
They can try to bribe regular people to do menial things. "Hey kid, I'll give you this VR game software if you buy me a burger from that vending machine".
They could also attempt to simply google it. While they might be naieve about how the shadows operate, they should know their characters skills and know that they can simply research these things. Criminal enterprise is an aspect of daily life for a lot of people and they have chat rooms. Data search is a skill for finding this sort of hidden knowledge. If the players want to know "What do I need to go buy a gun?" That's something they could get an answer for, if they had a commlink with a connection to the Internet/Matrix/whatever they call it now a days. And too much plaintext traffic over a cafe's public wifi concerning criminal enterprise will draw the attention of the authorities.
Making the connection
They might be caught and processed by lonestar just for hanging around the wrong part of town without a proper SIN. They'll be given a criminal SIN, and placed in prison/forcedLabor/collared/fined-and-freed. Prison is a great place to make contacts. They might owe organs to a bail-bond agent. Hell, the bail-bond agent might give them their first shadowrun job to pay off the bail.
They might actually find the contact information of someone with a criminal background while searching the matrix. Meeting them, developing trust, and then doing business with that person would be role-play worthy.
They could meet someone in the dirty dumpy section of town. "Hey, I need to buy a fake SIN" could be all it takes to get pointed to that guy in the corner that knows who to talk to.
I would leave it to them to come up with creative ways to overcome the problems that shadowrunners deal with out of hand: transportation, bypassing legality requirements, handling money, knowing who to buy from. I would throw some major conflicts at them through the course of their plans. I would have them suffer from someone abusing their naivety, but through that interaction gain knowledge of how things are done, and who not to trust. And eventually they upgrade from abusive contacts to people they can trust.