As much as I hate to suggest it, ebay might be your best bet right now:
http://cgi.ebay.com/Castle-Creations...item4cecc69d41
It would be nice if CC released lower kv motors; ~3500kv for 3s and 4s, ~2200 for 4s and 5s, and a ~1800kv for 5s and 6s. Otherwise, you really can't take advantage of the 6s capability of the ESC. The 4600 on even 4s is almost 70k rpm.