There is no known inhibitor for my protein, so i want to screen some drugs which are natural compounds and most probably commercially available, so for the drug library, which database i used to prefer.
If you're looking to do screening with real chemicals (as opposed to virtual screening), I'd look at the NIH/NCI: https://dtp.cancer.gov/organization/dscb/obtaining/available_plates.htm
You can get (for free) plates with a wide variety of natural products. They don't guarantee purity, or even compound identity, and just assume the folks who provided the stuff weren't fibbing. But it would still be a quick way to gather a lot of data rapidly, and you could look at related compounds from there.