Hi Oscar.
Ken covered most of the bases concerning procurement of the 801. I will add what I can and hopefully it will be helpful.
I was first introduced to the 801 atty by Don at a meet we did in Feb 2011. Their performance was so impressive, the first thing I did upon my return home was to go to Vaporkings and order the "bulk" 801 attys. They are inexpensive, work fabulously and I have never received a dud. Since then, I have tried the 801 carto as well and I was very happy with its performance. Madvapes has them in 1.7 and 2.5Ω.
I am a heavy vaper and enjoy throat hit very much. The slim auto batteries simply do not last long enough for me nor do the 3.7v batteries provide enough power. I use the 801 on a 5v box mod and it looks like an older cell phone with an antenna. Point being, with a 510 bat/801 adapter you can use the 801 on any battery you like. To tailor this to your preference, there is a "shorty" adapter which you could use to attach the 801 to a black, 350mah, 510-threaded battery. These batteries are also available from Madvapes. This will allow for much greater "vape time" and will also enhance the "penstyle" look as it will be even longer than the regular 801 battery.
Battery is here:
Mega XL 510 Manual Battery, Blue LED, 350mAh
Shorty adapter:
510 Battery to 801 Shorty Atomizer Adapter
You specifically want the "shorty" for a reason. In a standard 801 kit, the atomizer/cartomizer is wider and fits over the battery as it is screwed down. By getting the "shorty" connector you will be closer to the real thing. The regular size connector will be too large and create a very visible gap between the battery and the carto.
Filling the 801 cartos may be a bit of a challenge. The 510 "condoms" can't be used on the 801 and the tip, drip, shake and wait method will take quite a while. A syringe can speed the filling process.
All the best!