You will not find a reputable Pilates certification that is available online only. If you are not interested in completing the a certification in all the apparatus and are interested in mat only you have several choices that offer mat modules. I am personally familiar with and can recommend either Balanced Body or Stott Pilates. Other reputable companies worth checking out that offer a mat training include BASI Pilates and Peak Pilates. I am sure there are others that are also good.