you can usually find potassium sulfate at most full-service garden centers, or you can buy off ebay.
it's available in pellet or powdered form - either way 80#/acre is not much and you'll probably have to blend it with something else (fertilizer, sand, whatever) to get even distribution.
My soil tests indicated about 2 lbs per 1000 sq ft - about the same rate, and my garden is less than 1000 sq ft.. I got the powdered form and am going to apply in solution form in my handpump sprayer, half before setting the starts and the remainder a few weeks later. please note: K2SO4 is not readily soluable in water, requires hot water, lots of agitation and a little time. Don't apply directly to the plants, will cause injury.