If you have some lemon juice around, it's stable enough. Powdered citric acid is an option too though not many just have it sitting around. Worst case scenario use vinegar. Vinegar is the worst though as I'm pretty sure it influences the prevalence of anaerobic bacteria which are not what we want.
Best option is some legitimate pH buffer solution. Sulfuric acid can be used as it's one of the most stable and readily available (can buy a bottle at any auto parts store for around $10).
But as I said a lot of organic growers don't even pH before watering in, just an unnecessary step as the nature of organics do not rely on pH for nutrient availability to the plants. All the wonderful micro-organisms provide the nutrients to the plant. Trying to regulate the pH messes with these guys iirc. Watering in at 7.0 is better than 5.5, that's for sure in soil.
I'm no organics expert but I do know the pH range in soil is not the same as in other hydro or soil-less type mediums. I also know that most organic growers don't bother with checking pH as the soil will regulate itself. Perhaps someone who focuses more on that side of the fence can chime in. I'm a coco grower that doesn't really like the idea of organics for indoor grows. Recovery takes time in soil, much longer than in hydro or soil-less mediums in most cases.
One Love,
tobh