I won't spend time listing the dozens, even hundreds, of known entheogens ; there are excellent books on this topic available.  Your reasons for coming to this point are sure to vary widely, so I won't address each of them here.  Sometimes you find the plant, sometimes the plant finds you.  Either way, the next step is learning to live with this new acquaintance.

In my case, part of working on this path has been learning to grow and tend the plants I use.  This may be your situation as well.  However, some of our most valuable plants have controlled legal status and growing them isn't a risk you're willing to take (not that I blame you).  Also, not everyone's climate will be conducive to growing every substance you utilize.  It may just be plain impractical; not everyone has the space for a Brugmansia to grow to full height, or the ability to care for plants sensitive to climate conditions like tobacco.

There are some plants that I don't grow for one reason or another; these I collect when I need to, or will arrive in my life when I need them for a certain task.  There is great value in knowing where and how to collect the plants and fungi you need; the effort is part of the Right Action that goes into the job you do.  This Right Action is part sacrifice and part gesture of respect to the Grandparent.  It says you respect the powers you're approaching and are willing to do your fair share of the work.  It is also a reminder that we are not the ones in control of the effects of these plants (see Entheogens and Trance).

Any plants you grow for yourself, entheogenic or not, will give an invaluable insight into the life cycle of plants and help you understand the world from a plant's point of view.  Their voices will become familiar to you and you to them.  One of the best ways to honor the Grandparent spirits is to care for their children; doing this has great merit.  Working to create a communication with the plants will likely bring surprising results.  Plants often enjoy light physical contact, and may even actively demand it from you; a few minutes of daily attention from you pleases them and reinforces the bond.  Using a plant you yourself have tended and cared for adds an extra weight to how you use an herb in your work; imbibing of them in whatever form becomes a sort of sacrament and will foster a Right attitude of gratitude. 

Even if you only use a plant once, you will carry that relationship with you.  It may be necessary to continue to observe certain taboos; most often, a person is forbidden future recreational use of that herb.  Other action may be required, such as assisting others to know that particular spirit and herb, or giving ritualized attention at some time.  Whether or not a plant is physically in your life seems to be unimportant (unless they have required it of you); they are still part of your extended spiritual network.