I need to buy an air pump, so have been weighing up the options. But maybe the following may help to address your problem. Perhaps your switch just needs resetting. According to the Hakko website, these switches are relatively sensitive, and prone to cut-out if diagpraghm becomes faulty, or if the pump takes a bump.
Go to website www.hakkoairpumps.com. In the "Troubleshooting" menu there is a link to "resetting cut-out switch".
The website also lists their Dealer network in the USA, and I'm sure they'll source the part for you, and surely the costs to get it to you should be minimal.
A personal point of view: If a Koi Dealer in this country is the appointed sole distributor of the product in SA, then surely he should assist you in accessing the spare part for a pump he sold you for R4k - R5k. If he periodically brings in pumps then surely ordering a switch should not be too complicated? To go back to him and insist on support, would be my option.