I love Pet Shop of Horrors, one of my fave manga though I've sometimes forgot about it because it is hard to find sometimes.
Yep, it is possible to find it in bookstores like Barnes and Noble or Borders. Also, does'nt TokyoPop have it's own online store now?
Could also be useful checking out AnimeNation and Anime Corner Store.
if you need me, i'll be in the bamboo forest...
Seccotine (Spirou and Fantasio)- 80%
Sheeta (Laputa)- 100%
Moemura (Madoka Magica)-10%