You can secure the deal against the order value itself, but you'll need various forms of guarantees - speak to your client to find out if they're willing to sign these. However without capital backing and a financial record to speak of, you won't find a financial institution willing to take the risk of trade finance in this respect. Have you considered looking at other forms of finance to then take on the risk yourself? SMME loans? Capital finance through DTI? VC finance? SAIN network might help in this respect.
Can you not negotiate terms with your supplier? You might take a GP knock on extended terms, but at least you'll sort out the cash position and begin to build a reputable, financeable name. If you're importing, then you'll need to determine the supplier agreements in terms of FOB and the like, as it relates directly to payment liability and will impact on trade finance.
You can speak to a company called Blue Strata who are incredibly good at arranging trade finance for an entire deal to completion of cash changing hands, including the securing of preferential forex rates, but their risk management profiling is very strict and I'm not sure if your company would qualify.
Your best bet is to call and meet with as many trade and bridging finance companies in SA, not asking on a public forum. Alternatively, consider an SAIN drive...