Where to get plants?

     I'm trying to set up a plant tank for the first time.  I have the
CO2, the filtration, and the lighting, but only one plant.  i bought
an Amizon Sword for 3 bucks.(I dont even know if thats expensive?)  I
want to try out growing some plants for a while before I drop alot of
money into it, or set up a bigger tank with plants.  
     Is there mail order places for live plants?
     Can I get live plants out of the wild?
     (Would I want to?)
I would greatly apreciate the advice.  Thanx :)