There are two ways to get your hands on a Boost SIM:
If you are a Telstra customer and wish to change to Boost, then you’ll need a blank SIM which you can get from Customer Care. More info here.
Boost currently stock SIM cards to the value of $2, $10 and $40. If you want to join Boost on a $30 or $50 prepaid plan, grab a $2 SIM and recharge to the value you want.