How do I find equivalent tropical plants to replace the BD prep plants?

I would start by reading Wilhelm Pelikens descriptions of the prep plants in ‘Healing Plants’.  Then have a read of what RS says about the function of the plants in the Agriculture Course, so you can get an image of what you are wanting. Then any other author I could find.
Then I would read up some herbal journals about the healing effects of your local plants, and try and match them up. 
Part of RS approach is how plants effect the energetic activities, and this is what you need to match. That is the tricky part, having that sort of interpretation for your local plants. I imagine you could get pretty close if you just follow the herbal associations for both. e.g. Yarrow helping kidney and urinary function  > local plant that does the same etc.
Then once you have made your choices and made the preps, which may or may not mean putting them in a organ; I would get a trial plant, brassica or even possible a bryopholum ( succulent plant whose form will change readily) and spray both preps ( original and your new one ) and observe if they influence the plant growth in the same way.


You can also take the remedies yourself. Either just eat a bit of them and watch your response, or place them in a bottle , cover them with vodka, leave for a couple of weeks and then have a sip of the vodka from each and observe the effect.