Is it ever explained how the vegetation in Jurassic Park was created - the dinosaurs were explained by blood preserved in a mosquito, however before Alan Grant turns Ellie Sattler's head to see the first dinosaur, she is examining a leaf while remarking that it "doesn't belong here". I assume that this is necessary in order to create the ecosystem such that the dinosaurs can live in the present-day, but was it ever explained in-universe how this was achieved?
There is, however a brief mention in the follow-up novel that InGen maintained a facility on Isla Nublar that was tasked with the growing of exotic plants. Presumably much the same techniques were used to husband extinct plants and seeds (from amber samples), to engineer their DNA and to grow them to feed the dinosaurs.
In addition to its headquarters in Palo Alto, where InGen maintains an ultra-modern 200,000 square foot research laboratory, the company runs three field laboratories around the world. A geological lab in South Africa, where amber and other biological specimens are acquired; a research farm in the mountains of Costa Rica, where exotic varieties of plants are grown; and a facility on the island of Isla Nublar, 120 miles west of Costa Rica. - The Lost World: Jurassic Park
Notice that she said that the plant "doesn't belong here", not that the plant is an unknown species or should be extinct. It just wasn't where she expected it. Like finding a McDonald's on the south pole.
I infer that they migrated some plants from another region, presumably because they fit the dinosaur's habitat better than the plants that were native to Isla Nublar.
If the existence of the plants was more of a mystery (prehistoric plants, unknown species), then Ellie's reaction would probably have been a bit stronger.
Going by that interpretation, "by boat" would probably be the most accurate answer.
Like the dinosaurs, the scientists recreated the plant life from the dino age. It really isn't stated how, the dinos were recreated by filling in the missing dna with frog dna. Perhaps the same was done for the plants, using similar species that exist today.
The only way to recreate the vegatation they would have to find the DNA of such plants first. For the dinosaurs they found it inside mosquitoes. For the plants, any bugs that feeds on plants and preserved just like the mosquitoes were would do.
If I recall correctly, none of the movies state that they have found bugs and were able to clone the vegetation DNA.