I first found out about these bags when Silvia Tcherassi for Payless shared items apart of her collection outside of Payless on the Martha Stewart Show. After that I waited, thinking that she might incldue similar designs in her Payless collection...but, still nada:(
But, recently I've been seeing these Mochila designs popping up everywhere from Club Monaco to J. Crew. And the styles are really cute! Plus, back in 2010 I found that Honestly WTF had written about these handbags and provided options for where you can purchase the handbags onlne.
The Club Monaco Mochila
The J. Crew Mochila
Update: Just saw that Net-a-Porter has Mochila bags on the site too!