Get a cheap ie free one from somewhere. Then take it to BigW and exchange it for a new one and pay the refill price.
Ive done it a couple of times now. (some stores wont accept old cylinders, others do)
Get them at Garage Sales Vinnies ETC I have bought about a Dozen Roughly $2.00 each..Some new some old but exchanged the old ones for new..Paid $5.00 for one Sodastream BUT it came with 4 new cartriges..
Cheers
PJ
but by the time you've spent 30-50 bucks on a sodastream bottle and adaptor, you've committed urself to the 2nd most expensive ongoing CO2 option available to you.
Have you considered to start kegging by doing primed carbing in the keg and using a keg charger to dispense? then when funds permit upgrade to a real bottle or a fire extinguisher?