You need a real fertilizer supplier, supplier to growers. They probably won't have langbeinite, they will have an equivalent. 22% K, 11%Mag, 11% S. You'll have to look at it carefully to make sure you aren't getting straight Potassium Sulphate, 0-0-22, because sul-po-mag will also be tagged 0-0-22.

langbeinite or not, these are mineral salts and so should be added with good amounts of OM unless your soil is quite high in percent OM, say over 10-12%. Not more than 200 lbs per acre in any case, per year, which is very little per sq yrd!

I couldn't find it anywhere either; I just ordered Aggrand's Kelp & Sulphate of Potash in desperation. I am hoping that it will serve almost the same purpose in the garden.

Here is a link that might be useful: Aggrand - kelp & K

I would think sulphate of potash and magnesium sulphate (epsom salt) will achieve it.

That's what I'm hoping for (crosses fingers for luck)

Sul po mag is great stuff if and only if your soil test shows deficiecy in sul, po, and mag. I've come across this only once, in a situation where grass was turned over to vegetables. Regards, Peter.

There are different paradigms regarding how much constitutes "desired" levels of any mineral.