Finally, if all else fails, you can obtain native plants from wild
sources. This isn't something I recommend, though. Collecting
plants from the wild can damage delicate ecosystems. Furthermore,
it can contribute to the extinction of endangered plant species -
if you take a rare plant out of its native habitat, not only is
that one less plant in the wild, that's also one less plant that
could be reproducing to regenerate the wild population! Frankly, if
wild harvesting is your only option for obtaining a certain species
of plant, you should really be asking yourself if you really need
that particular plant in your garden. 

However, if you absolutely can't be dissuaded from wild harvesting,
you can at least take these tips on how to minimize the damage you
do:

1. Never collect on public land. Leave it where it is for the
   public to enjoy.

2. When collecting on private land that you don't own yourself,
   always obtain permission from the owner of that land.

3. Know the conservation status of the species you want to collect.
   Don't be a dickwad and take something that nature can't spare!

4. Instead of digging an entire plant up to transplant somewhere
   else, take only the seeds or other reproductive material. Not
   only will this leave the mother plant in place to produce more
   offspring, this also helps prevent the spread of soil pathogens.