It's common knowledge that an underground economy exists in prison where cigarettes are a common currency to exchange for commissary items, like snacks, or contraband, like porn magazines. When a prisoner is placed in a Special Housing Unit, isolated from the rest of the general population and held inside a small cell alone for all but an hour or so each day, if that, access to goods is even more difficult and restrictions around commissary products are tight. At the Marshall Project, Matthew Azzano reports on "how people in 'the box' use their ingenuity, collaboration skills and a form of 'fishing' to get what they need." From the Marshall Project:
Stamps are a major form of currency in the box because virtually everyone needs them, and they are small, concealable and easy to transport in any quantity. The most notable product missing from solitary confinement commissary is tobacco. Going straight from smoking regularly in GP to being isolated and empty-handed in solitary, many prisoners' first question at the rec pen is, "Who's got the 'bacco?'" Accordingly, rolling papers, matches and lighters are valuable as well[…]
Whatever you're buying or selling, you must learn how to "fish" to participate in the solitary economy. Fishing lines are made of ripped strips of sheets, towels and clothes. With thin enough strips, you can make about 100 feet of line out of a single state-issued bedsheet. To fish, you attach the item you're selling to the line along with a weight, such as a bar of soap. When you toss the line toward your buyer, the weight creates the momentum needed to carry your item through the air and drag it into the right cell after it lands.
Of course, commodities can fall off the line or land in places where they can't be recovered. Arguments can ignite over accusations of theft, so you need to know who you can trust to pass on your goods. In my reporting, I heard about one guy who didn't have any family members on the outside to help him pay for basics like toilet paper. Thanks to his cell's central location and his good aim and trustworthiness, he was able to support himself by securing other people's lines. It started out as something he would do to pass the time, but he ended up getting really good at it. People would look out for him, sending him items in exchange for his help.