More than likely I will purchase mine from here
. This is the same place where I get my bags from.
If you live outside of the US, you might be able to find a closer place, but this is the closest and cheapest here in the States.
There are some cheaper places on Amazon
, but there are no reviews. I don't know if they use cheaper cells, which relates to shorter lifespan. The one at Total Vac are from Karcher.