As luck would have it, Anna Maria Horner announced today via her newsletter that her needlework collection is now available in her shop! 

Here's a screen shot right from Anna's needlworks page.  Each set of pearl cotton contains 9 balls, which works out to about $4.75 each.  I pay $5 each at my local yarn shop, so that sounds about right to me.

A few readers also shared online sources for pearl cotton sold single.  One reader has already done some shopping at Yarn Bazaar, who sells an impressive selection of Finca pearl cotton, size 8.  Sometimes they have a color card available, but they seem to be out of stock now. 

Another reader recommended the website of her local quilt shop, Nordic Needle.  They have a large array of DMC pearl cotton.

Hope that helps!