SAPIM makes good spokes but that doesn't mean that all of it's styles and configurations are freely available. Those wheels have straight-pull spokes. Straight-pull is a spoke without an elbow - the commonest type. Bike shops don't stock straight-pull in various lengths and have to order them in specially for a job. A box of exotica is typically 72 spokes and about four times the price of a similar, standard, elbowed spokes. The dealer then orders a box of spokes, makes you pay the price of the entire box for the three you need and keeps the rest in stock for ever and ever. He/she has to do it in order to at least break even on the deal. There is virually no chance that they'll ever use the rest of the box. This is because a) the spokes are straight-pull and seldom used, b) black instead of silver and c) flat instead of round and d) very short, thanks to the deep section rim and radial lacing. The only time they will sell that length again is when another customer with the same wheels requires any.
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So, strictly speaking they are not proprietary but stock. However, it is a rare item and breaking one will leave you without a wheel for weeks. True proprietary spokes are such as found on Mavic's Ksyrium wheels and the ultra-stupid Spinnergy Spox wheels.
The best wheels are ones that use regular round spokes with elbows in a 2.0mm, 1.8mm 2.0mm butting configuration. Some manufacturers have now gone and done something incredibly stupid and added a 1.8mm - 1.6mm 2.0mm spoke to their line-up (DT Swiss) and the likes of Specialized uses it on their Roval branded wheels. This is very problematic since a single spoke can cost you a staggering amount, for the reasons given above. Customers often feel compelled to not consider alternative replacement options because of a further complication - nipple colour.
Having said all that, if forward thinking is not your obsession, go for it. I'll admit, those wheels sure do look fast.