You may still be able to get your hands on a 9.1 remote agent license somewhere, but you should be fine using a v10 one instead, as I believe the agent licenses all work with older versions of the server software.
Best bet though to be certain would be to contact customer services, and check with them to make sure. Contact details are here :
http://www.symantec.com/techsupp/home_homeoffice/contact_cs.html.
They should even be able to confirm the exact part number(s) you need, which with the number of different variations around is handy, as I know personally they confuse the hell out of me!