Visas are required by all and cost around US$120 for a 3 month visit and take about a week to be issued. You may need to provide an invitation and other documents.
Visas are readily available at Nouakchott International Airport without the need for an invitation.
Visas are allegedly available at most border crossings for 30 euros, valid for 90 days. Try and get your visa in advance or check the local situation if you are crossing into Mauritania overland.
Visa Information for Mauritania