Well, Resveratrol is Resveratrol no matter which company sells it as long as that company is legit and has good quality. It is a fantastic antioxidant that is also good for cardiovascular health, but that’s about it.
You can get it from any health food store (GNC, Whole Foods, etc) and many other places, but I would be careful about online ads for it (and the "free trials" are hardly ever free since they have to get you to pay for it somehow). Unfortunately, Resveratrol is one of a few high powered antioxidants that are fads right now and when there is a fad, there are always people and companies that take advantage of that popularity and sell their own piece of it. The unfortunate part is that too many of those people/companies make baseless claims in order to sell their product faster and for higher prices.
It’s best to get a product like this from a company that you know and/or trust. If you’d like, I can also point out a few companies out there if you can’t find any in your local area.

As far as dosage, it depends on how much your body can use. Just like different people need different amounts of water and different amounts of food, each person needs different amounts of various nutrients. That being said, the most popular dosage that people order from us is the 30 mg and 50 mg doses, but you should be able to find those in any store as well. Good luck and I hope I helped!