When I needed a cable like that for my active speakers (JBL LSR305) I used a 3.5 to rcax2 I had on hand then made separately two rca to xlr cables. I'd try the simplest solution first, using an adapter type cable between the dissimilar ports. As was mentioned, the speakers may have an XLR input but may not be balanced otherwise, so don't sweat it too much. How long a cable do you need? Unbalanced should be fine unless you have need for a very long cable....
ps Monoprice has a lot of choices, too....or even Amazon.