Propane - Putting liquid propane in a vapor canister?
Propane liquefies under pressure.  All propane tanks hold liquid propane.  The pressure is maintained by a small amount of the propane that turns back into a vapor in the tank.  Propane appliances draw off the pressurized vapor, which reduces the pressure in the tank, and more of the liquid vaporizes to replace it.  

If you read the description on your link, you get the tank filled at any propane refiller, and it's refilled with ordinary propane.  

BTW, that looks like a regular propane tank.  The thing that can vary is the connector.  If you take your weed attachment to a hardware store like Home Depot or Lowes, you're likely to find that it fits one of the standard connectors on their tanks.