The "fixed prefix" mean that the ISP provides each client with at least a /64 prefix, preferably a /56 or /48 (this is the equivalent of "fixed IP" for IPv4).
Most professional ISPs do this; those currently known to do this are:

1. Nerim (France)
2. FDN (France)
3. Free (France)
4. Internode (Australia)
5. TekSavvy (Canada)
6. Andrews & Arnold (AAISP) (UK)
7. Tiera (Russia)
8. XS4ALL (the Netherlands)
9. OTE (Greece)
10. Forthnet (Greece)
11. [..] Please complete with ipv6 help pages from IPv6-friendly ISP

1. RCS-RDS offering PPPoE with /64 P.D. (Romania)