Where to find motorized bike supplies?

[admiralblue16]

so im trying to make a motorized bike that i can still pedal if i turn the motor off so my dad said i should make a fan belt one and so he said i need to fins a sheath/pulley/drive rings i found one on a website for 40 bucks but i wanna fins a cheaper one. a fanbelt a throttle twist grip and the rest i can find