The "easy" way, for you as a "Thematic" collector,
is to go to your
ebay,
Enter the text "Thematic Flora" in the "stamps" category.
Go window shopping on the result, and pick what appeals to you. "sets" will be advised.
..or "topical Fauna"
Be aware, a collection in this manner, will end up 4 times more expensive,
than collecting "worldwide" lots, and patiently building a collection.
I see for $2,000 you can purchase 7 stockbooks full of Flora,
and become an immediate advanced collector, but do you want to go that route?