Omni Plant seeds are acquired by completing an Opportunity, just like how you got egg, cheese, steak, and patty plants. I believe the Opportunity is a request for 10 Outstanding homegrown Steaks, and you need to be level 10 in the Gardening Skill before you'll get the call.
Forbidden Fruit you get from the Science skill. By using seeds (any seed will do) in a Gene Splicing Experiment at the Science Research Station, you have a small chance of transmuting the seed into a Forbidden Fruit seed. Unfortunately, the plant only gives you one Forbidden Fruit before going barren, so you'll need to splice more seeds if you want more than one Forbidden Fruit.