Why not buy old formalin - how is it ever to go off?
All things need persepctive. I do alt process, and the gelatine made up and stored before actually coating is preserved with a small dose of formalin. A little goes a long way. 20ml per litre, and it causes cross linking to harden the gelatine as well as preserving it.
Back in 2004 I bought a 1L bottle for formalin, and have maybe used 60mL to date. It is only openned under the exhaust hood, and is stored on a high shelf and is well labelled.
All photochemicals I keep in the house I have MSDS print outs for. The binder sits outside of the darkroom, and I have explained to my wife what it is and how the information is organised.
She has a certificate in Occupational Health and Safety Sytems, and has a perspective on what the risks of my hobby are.
She understand when at times she knocks on the darkroom door (the fan of the fume hood makes it hard to be heard talking though the door), and I answer after putting the caps on the chems of concern, wearing a lab coat, nitrile gloves, closed toe shoes, and a respirator with filter pads, and a full face mask that I am doing the right thing.
I suppose it helps too that she and her friends love the hand made soaps that I make, and that entials buying hydroxide 50kg at a time.