I bought my case at Health Cabin (healthcabin.net). It fits one 510, fully assembled. However, shipping is expensive, since they ship out of China, so you might want to keep that in mind. It's a nice little case, since it fits in a pocket or purse.
However, you have to be careful how you put it in the case, since it's possible to press the battery button and keep the atomizer going. I always listen to my case after I shut it to make sure I haven't done this.
I haven't seen the type of case anywhere else that fits exactly one fully assembled e-cig.
In general, people seem to be fond of their PCC cases, since those fit more than one e-cig and will charge the battery (as long as you keep it charged). However, you must take the battery off the atty/cart so that it charges.