co2 påfyllning

Hi,
Apologies for the english :-)
I've been using soda stream for co2, but get through them fairly quickly so was wondering about getting a bigger cyliner.
I can't see to find anywhere that fills them up that isn't really far away.
I thought 5kg or 10kg would work, but local fire stations don't fill them, and most websites seem to be a swap and we'll only give you prices ones you register (company only?).
I live in Tumba - south of Stockholm.
Anyone have any tips on getting more co2?