I'm in the process of setting up my first home brew and TAB's system at home. But I'm quite discussed to find out how much CO2 3.5 or 6kg tanks are! Will it be a problem to buy from US and will it be possible to fly it over?
If you're getting it sent here I would say that postage would be more then the bottle. If you are going to the States and are going to bring one back in your luggage, from my understanding you will have to be IATA licensed and fill all the paperwork out.
why don't you rent one or just bottle condition your keg? There are lot of forums about it on this site.