You do not always have to buy cullet for casting or billets - you can make your own. Billets lead to less veiling and bubbles than just putting in your old cullet into the mould.
Create a mould by using dams or pouring investment materials around something like a plastic salad tub to make a billet – the shape is not usually critical. Place a reservoir such as a terracotta flower post above. Take the temperature to the 650C – 670C region for a 1- 2 hour soak followed by a long soak at 830C.
Normally, the higher you allow the glass fall, the fewer bubbles, but you're usually limited as to how high you can go in the average glass fusing kiln. You'll get some bubbles, but if you then put your new billet as a single piece in a reservoir for your casting you'll get the second flow that removes more of the bubbles.
All the glass must be thoroughly clean before being put into the pot for making the billet. Do not use iridised glass as it reduces the clarity of the billet. Do not use glass that has been ground as that will cause hazing in the billet. Instead, cut off the ground sides before washing the remainder and including in the melting process. Do not include the ground off-cuts.
Based on information from Cynthia Morgan (Morganica)