I'm trying to make some flux for soldering.
Let’s clarify what you mean by “flux for soldering.” Based on the
ingredients you are seeking, it sounds like you want to make Prip’s
flux–which is great. It’s just that if you have too much trouble
finding those ingredients and want to try an alternative, we should
be clear on what you are trying to do.
I know you may well know this, but since you didn’t say: Prip’s flux
is mainly used to protect sterling silver from firescale. I’ve heard
(though I don’t know from experience) that it is not very good for
fluxing the actual solder joints. So what many people, including me,
do, is use paste flux on the joints, and Prip’s flux, or some
alternative, on the rest of the piece.
BTW, boric acid is widely available under the brand name
Roach-Prufe. It’s not 100% boric acid, but close enough to make a
good flux. I’ve been using it in my homemade Prip’s flux with very
good results: as long as the metal is coated thoroughly and evenly, I
get almost no firescale.
If you don’t have a hardware store in your area, Home Depot or
Lowe’s might have the ingredients.