I bought my sodium ascorbate from Amazon. They had several suppliers; I bought the cheapest (per weight of product), since it's all supplement grade, therefore chemically identical. You can also search for "buffered Vitamin C" -- that's one of the most common descriptions for sodium ascorbate. As you note, you could make it yourself by titrating an ascorbic acid solution with sodium bicarbonate (baking soda) until no more effervescence is observed, but then there's the issue of knowing how much you have in the resulting solution (yes, possible to record everything as you go, but not as easy as just weighing out a chemical).
For "equal" quantities -- I presume that's equal molar count. One molecule of ascorbic acid plus one molecule of sodium bicarbonate. Repeat 6.02x10^23 times, and you have a mole of each. You'd have to combine them in water solution for them to react, but calculating the molecular weights would allow you to know what weight of ascorbic acid and what weight of sodium bicarbonate (don't forget to correct for water of crystallization, if any -- anyone know if baking soda is anhydrous?) to combine to give the right final amount of sodium ascorbate.
Or you could just add vitamin C in powder or even tablet form, to a corrected quantity, and then adjust the pH of the final developer solution by adding more borax or a little bicarbonate.
I found it easier to spend $14 or so for enough sodium ascorbate powder to last me years. And Amazon brought it right to my door.
