This month I am going to put down some 0-0-50 granular on some of my mango trees. Muriate is same as Potassium sulfate. The granular potassium sulfate is what you want. Though Har's (Guanabanas) potassium fertilizer sounds like the ultimate because it includes minor elements/trace elements..
Universal Supply in Broward gets the granular from time to time. I got two bags last year by random chance.
Address: 2171 Blount Rd, Pompano Beach, FL 33069
Phone: (954) 979-0600
The potassium sulfate powder is what is put into irrigation systems aka fertigation. It is highly dissolvable. You could use this powder on your trees as a surface fertilizer but would require frequent small doses.
Aside from Har's formulation the only form of potassium you want is the sulfate, with granular the best and the powder being usable if you are disciplined enough to hit your trees with it, small doses each few weeks. If you have lots of mulch around your trees the K2SO4 powder might chelate into the mulch and change to slow release.