Venusaur learns Sleep Powder at level 15, but assuming you are using a legit, evolved Venusaur, you can do one of these two.
1. Train an Ivysaur to learn Sleep Powder at level 15
2. Have the move tutor teach it to your Venusaur (I don't think move tutors exist in Pokemon Red/Blue/Yellow)
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Bulbasaur learns it at level 41. (Ivysaur learns it at 46 and Venusaur learns it at 55)
Venusaur Learns:- Growth Lv.43 Sleep Powder Lv.55 Solar Beam Lv.65
Bayleef Does Not Learn Sleep Powder in HeartGold/SoulSilver
Butterfree learns sleep powder at level 15.
Shroomish doesn't learn Sleep Powder.