We can get citric easily enough at a proper chemist.Thats what I use, and it seems very effective.
I have 5l of sulphuric, but of the two, I find citric better.
Best of all in that line is to get the metal really clean, and to solder when it isn’t too hot.,and use a lot of flux. That way water is pretty good and leaves nothing behind. You just don’t get any of that burned on black glass – its all a soft paste which washes off in a couple of seconds. probalby wouldn’t work on a big job, but on small stuff I rarely need the pickle.
Ordinary chemists also stock caustic which also works well.