Resource: Rolleiflex 600x series battery cell upgrade
This company in Germany offers a cell upgrade against the well-known problem of Rolleiflex 600x batteries loosing their ability to remain charged. Price is €38 (~$49) incl. doing the work and they seem a qualified battery seller. The eBay DIY kit is $39, so it is quite attractive.
Site is in German. No relation, just a discovery. Have not used them, but comments on the site and German forums are positive.
Just FYI for people in the US: These guys do the rebuilds as well. $98.
Rollei & Hensel Service
1275 Bloomfield Ave. Building #8 Unit #68 Fairfield, NJ 07004
Business: +1 (973) 244-9662
Business Fax: +1 (973) 244-9817
Have a completely equipped worshop, and both used to work for Rollei USA and have decades of experience. They are not going anywhere any time soon.