What else can be used as flux instead of borax for silver casting?
I would suggest charcoal and no flux. Keep molten time to a minimum. Charcoal is used on molten lead for long periods to reduce oxidation. For many copper alloys , hydrogen absorption is a problem ; it evolves during freezing causing porosity. I read in ASM Metals Handbook that molten silver may absorbs oxygen causing porosity. Charcoal should provide some protection from both H and O..
Borax seems pretty easy to get in the US. However, long ago I mixed flux to make a slag for copper alloys , it primarily contained : soda ash ( sodium carbonate), beach sand ( silica), and fluorspar. There are a number of sites on the net that sell proprietary fluxes ( for copper alloys). And one site gives info on the flux
 components -  911 metallurgist.