The way I see it you have three choices for that design:
Sheepskin (natural top and bottom)
Sheepwool (natural top, synthetic bottom)
Fake fur (synthetic top and bottom)
Sheepskin is the most expensive as it is the actual skin of the sheep after it has been slaughtered, sheepwool is cheaper as it is the sheared wool of the sheep attached to a synthetic base and fake fur the cheapest is made in a factory and usually used in cheap slippers like the grandpa slippers you say you don't want to make. Any slipper made from anything less than 100% sheepskin will sell for less than the real deal. Then all you will have is a cheaper alternative rather than a comparative product that is cheaper. You can look at reducing the cost other parts of the slipper such as using a cheaper sole but that is such a negligible part of the overall cost compared to the sheepskin that it might only shave a tiny fraction off.
The reason it's so expensive is that it is quite a labour intensive matter to cure, clean and process the raw skin and not many people do it and those people usually have buyers lined up whether export, medical use, rugs, underblankets, slippers, car seat covers etc. (although I don't know if many people still use it on car seats outside of pimps and drug dealers). It's not like offcuts is readily available as the smaller pieces as even small pieces are turned into stuff like bicycle seat covers.
Even if you can procure under current circumstances at a lower price, it won't last as soon as things get back to normal, you won't be able to get it at same price and your cost advantage will diminish and ultimate your businesses whole value proposition with it.
I know more or less what sheepskin cost at tanneries and it's not cheap. You might want to look into other aspects of your business such as marketing as it's not something that you can just buy from the abattoir and process yourself the equipment, space and time required to properly process the skin is beyond the effort a manufacturer will take (unless this looks like something you want to do). However if you are making R100 per pair and you can make and sell enough in a day then you might very well have a business.
Ask @Admin top make the footwear page (https://www.sb2sb.com/draft/footwear/) live over here and post a business lead to see if you can get a constant supply at a good price.