The Futaba R606FS receiver has been discontinued because it's not compatible with all FASST systems. The R607FS is being substituted in the 6-channel systems.

Here's the replacement battery pack for the 6EX transmitter:

FUTM1440 NT8F600B TX 9.6V 600M 4YF 6EX US$36.95

or

FUTM1462 NT8F700B TX NICD 700MAH 3PK US$39.99

These parts can be ordered through any hobby retailer who sells Futaba systems and accessories.