You can't go directly to them, you have to buy it via an ISP. I would strongly advise against both Web Africa and Axxess - they have abysmal customer support. Afrihost's social media team actually do a very good job of chasing up customer support, so if you have no option but to use Cell C's crappy product, get it from Afrihost. Also, get the smallest package and DO NOT take up the offer of a "free" modem. It's not free.
I have been on Rain since the launch last year and it has been pretty good, except for one week where we were getting zero upload speeds, which in my line of work is a big problem. That's why I got the Cell C card to use as a back up since I ditched my ADSL line completely. It worked pretty well at first, so much so that I decided to upgrade it to the max 200GB package and I downgraded the Rain to their smallest 25GB option.
Since the beginning of this month the Cell C network has been rubbish. It's so slow in the evening (0.6Mbps) that it is not usable for much other than email. I swap out the sim for the Rain one and I am back to 1st world internet (avg 30Mbps d/l). Nobody is able to explain why Cell C went so bad. The obvious answer is that they have oversold the capacity, probably since Afrihost started selling it.
Anyway, maybe you'll get luckier than I did.