To buy from the market, you must be inside a market, which means inside a Commercial Centre facility or a Trading Station (1 or 2), or inside Centrepoint Space Station.
Then by looking through the Traders Lounge in the RPG centre you'd find this link:
Which you can copy and paste because I apparently can't use html as every time I try links just disapear.
I'm not sure how often thats updated so it's best to check with whoever supplies access to the facility or station. You can also find someone with access to a market to buy for you, however this carries it's own risks if there not a reputable middle so it's best to access it from a friendly facility where people won't arrest and execute you for fun.