In order to access the Nightmare market and therefor an onion link, you will need a software called 'Tor'. Once installed, 'Tor' works just like any other search engine. In the top search bar you copy-paste the official market link: http://nightmareocykhgs.onion. Always make sure that you are on the correct site! Now you should be on the market site and will be able to search and find whatever you need!