Your best bet on the northside is that brew on premises place in virginia/Zillmere. It's called brew 4 you or similar sounding, and it is on Zillmere road, on the left hand side if you're coming from Sandgate road.
$20 for the 2.3 kg bottles, haven't filled up my bigger one yet so don't know the price.
I think it's about the only place that refills on the spot, they swap as well, but I'm like you and want to keep my bottle. Make sure you stick your bottle in the freezer the night before and keep it cold while you get there, that way they get more Co2 in there.
Florian