Is it possible?

I have 3 cacti which were all doing really well until I noticed lots of tiny bugs surrounding them - long story short I identified them as fungus gnats and their larvae have completely eaten the roots of one of my cacti. The other two are looking worse for wear I've repotted them and lots of forums advise the use of hydrogen peroxide to kill off any remaining larvae, but I can't seem to find any. Is there an alternative I can use, or anywhere in particular that would sell it? I've tried my local pharmacy but no luck.

Thanks in advance!

I don't think you can put fungus gnats and cactus in the same sentence. Fungus gnats need two things to survive: damp soil and fungus. Usually, the larvae are feeding on fungus and rotting material (including rotting plant roots) in the soil but if they get desperate, they will bite healthy roots creating an opening into the root for a fungal infection.

But... cactus require really dry soil. The fungus gnat larvae will hibernate until conditions are better for them but I have never heard of them infesting cactus. Is your soil staying too wet for too long?

Insecticidal soap used as a drench will kill the larvae as will spraying the surface of the soil with full strength rubbing alcohol (70% isopropyl alcohol). Please note that rubbing alcohol and drinking alcohol are not the same thing.

Yes, try watering less often, also provide more light and airflow to speed up the drying out of the soil. You can use sticky flypaper to trap the flying adults (put it near soil level) and that soap drench (use a product made for plants if possible) sounds good. You can also try a biological treatment (Bt is an organism that people use to control mosquito larvae and it works on fungus gnats too) or insecticide (imidacloprid into the soil is effective), but I would stick with the simplest approach (soap) first.

My baby cacti do bring on the fungus gnats sometimes, so those two words are not mutually exclusive. The key word in that sentence is "baby" because I have to water my very young seedlings much more often than adults to keep them happy. I do not have fungus gnat problems with my adult cacti, because I allow the soil to dry out all the way in between watering.

The fungus gnats are there because the soil is staying too moist and the roots are rotting. The gnats are simply a symptom of a larger problem and that is rotting roots caused by soil that is staying too moist, not by the gnat larvae.

If you allow the Cactus soil to dry out very deep into the pot, which is best for the Cactus, you will deprive the larvae of the moisture they need to survive and you may have a chance to save your Cactus from root rot. No further treatment of the gnats is required once they have been deprived of damp soil.

When you repotted, I hope you used a very porous potting mix and a relatively small pot that will dry out quickly and thoroughly.