What else can be used as flux instead of borax for silver casting? I am new to jewelry. I would like to try casting silver (pure) in so called green sand. In my research I see that it is recommended to use some sort of flux while melting silver. The most popular and widely available without a doubt is borax (sodium tetraborate if I am not mistaken), however it seems to be for some reason (toxicity maybe) illegal where I live and I cannot get it that easily. 

Are there any common materials that can be used as flux for silver melting instead of borax? 

I have seen sodium carbonate (wash soda/calcined soda) being sold as some sort of flux and I have read something about sodium hydroxide (lye) being used as flux in some applications. Would these materials be suitable/better than nothing for silver melting or should I not worry about it at all and pure(ish) silver should not oxidize anyway? Or is there anything else that might work?
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.