see the vodacom thread http://mybroadband.co.za/vb/showthread.php?t=70963. Both MTN and Vodacom have contract offers that include either rand value, airtime or data and the data card, and cost substantially less that purchasing a data card outright. The devices are not network locked; i.e. you can purchase a VC card and use it with a MTN sim, or vice versa. If you have good reception in your area, I would go for the Huawei E220 (USB device). If possible, borrow a unit from a friend and test it first.
If you really need a nonMTN, non Vodacom card, they are available. A fairly recent thread covered the topic, but I cannot locate it now. Expect to pay between R2100 and R2900.