I bought agri strength from the local weekly cattle market, you could try there if there is one near you, or farm supplies on the internet.

Be careful though where and how you use it, it is very strong.

I am not saying you shouldn't use strong weedkillers but offering an alternative perspective.

Hand-pulling spikes and always covering bare soil with landscaping-fabric is a slow method but is more soil friendly. Never try to dig it out as you may spread it as tiny root fragments can grow into new plants. And cut off the spore-forming spears in the spring before they ripen - they look a bit like asparagus.

Normal Round-up type weedkillers (glyphosate) can kill it but are not that effective because of its impermeable cuticle. What often happens is that the competitors are killed more effectively giving Horsetail free reign and making the problem worse.
Which brings me onto another (slow but organic) way to control it - allow the area to be fallow but keep it mown. Horsetail doesn't compete well with grass in a meadow environment and the mowing starves it out.

I have recently been told that it will wander around a site over the years, die away in one place and appear elsewhere. It's not that pernicious but still a pain in the proverbial.

I know all you gardeners poo poo my method of getting rid of mare's tail, and that I'm still just a beginner, but I've always done it this way and for the last 18 months I've been sticking a cane in to mark the spot and NOT ONCE has it appeared within 2 feet of that place.
I still cut it off just above ground level and give it a really good dowsing with malt vinegar.
Maybe I'm just lucky but will somebody just try this to prove my case and let me know the results ?

Anythings worth a try Hank and much better than chemicals.......

Maybe mine wasn't that sort Snoop. It was a 5L container of glyphosate with instructions for large quantities....I dilute it well but it certainly worked.

True Snoop, at least its less harmful than some of the old weed killers, nothing grew for years after them....except mares tail perhaps, it has after all been around since the dinosaur era. :0)

Membrane isn't foolproof - a friend if mine tried it but the blessed stuff forced its way through it!