Failing getting hold of enough of the original plastics, there are some very, VERY nice proxies available from Vanguard Miniatures (for all your infantry needs, inculding those elusive Terminators! Plus very suitable models for Thunderhawks, a few rhino variants, land speeders and even bikers). Look up the Tusculum Nova/Novan Elites range. You also mentioned Imperial Guard - other parts of the same (and cybershadows) range on there have been used by several people on this board (with more talent than I) to produce some very lovely Imperial Guard regiments.
It's worth a look, I'd say. Also, a suggestion - mention your general location, might help encourage respose if people know they're in the same part of the world and all that.
Good luck with the Marine hunt!
