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I have never once had any trouble finding rubber trees, but if you do have trouble look into IC2's crop system as you can get sticky reeds which give rubber.
Slimeball to resin is Gregtech thing. I tried to start world without rubber trees once, but unfortunately disabling generation of rubber trees also disables centrifuge recipe for propolis which made it impossible to do anything with parts of IC2 requiring electricity.
Edit: maybe not that impossible, I completely forgot about stickreed.