There are a number of eBay vendors who sell small quantities (usually 18-20) of butted DT spokes for <$1 each.

I've bought from this vendor http://cgi.ebay.com/DT-SWISS-COMPETI...item3ca83904e4

The only hassle is that eBay doesn't easily allow you to tell the vendor that you need 289mm length, so you end up having to email the vendor asking if he has the size, before you order.