it has been brought to my attention that sometimes it is difficult to locate balls of perle cotton threads- seems to be a (location matters) thing- some areas just are into more (modern) things- a couple more (brands) are: Valdani; Finca; and Anchor
a couple sources are:
ThreadArt.com
Pieceocake.com (they sell variety packs which are kind of nice to get)
CraftConn.com
herrschners.com
to find more- browse (needle work sites - like cross-stitch /needle arts suppliers)
i am lucky enough to have a couple lqs's that carry handdyed/felted wools & threads to work with them. I have seen sets of perle cotton balls at places like Joannes, hobby lobby & ben franklin crafts.