I looked for a long time...I purchased a Panasonic KX-TG5571 with 2 additional handsets and I'm absolutely happy! The set allows up to 8 handsets. I love this thing. The talking caller id is entertaining as well. Still a rebate on them as well. Best price I found was from this business on Ebay....they are termed as "returns" but I'll be damned if they aren't new...they sure as hell look and function like it. Complete with full warranty as well.
Here it is with one extra handset for a bit less. The additional handsets go for $49 and a $10 rebate.
Item # 5850217049
http://cgi.ebay.com/PANASONIC-KX-TG5571M-5-8-GHz-Phone-1-KXTGA550-Handset_W0QQitemZ5850217049QQcategoryZ58356QQtcZph otoQQcmdZViewItem
Staples has them on sale right now for a good price as well.