Hi Brett,
Purchase are not possible in SA, no - there are some people that will sell you in, but they usually bring it in from Europe and you pay a small fortune for the device only as it is more expensive in Europe and the States (in the States they are currently giving minis away for free to residential users!)
I purchased a Mini online through Starlink last year, delivering to the first camp where I stayed in Livingstone - the unit arrived at DHL in Livingstone within 8 days (from the States) and they agreed to keep it there until you can collect.
Starlink will work in SA but for how long varies - some get cut off after 2 hours, other 2 weeks and some get 2 months. But once you have used it in SA, if you leave and return, it immediately gets restricted.
There is a very informative "Starlink Users South Africa" Facebook page - I am regurgitating what I have gathered from there :-) I only use mine to travel in other countries.