First if your unsure of your zone:
http://garden.org/nga/zipzone/The 3 big ones:
https://www.eburgess.com/324-seedshttp://www.burpee.com/http://www.gurneys.com/category/flower-seeds-and-bulbsI use eBay a lot anymore for seeds also and I've had great luck with everything I've bought so far, especially if I'm looking for something rare or hard to find.
The best way I've found to search about a particular plant is google...USDA has an invasive plant list but it's severely lacking...trumpet vine for example is very invasive here in Pa. (don't ask how I know that), but necessarily not so elsewhere...pampas grass, lily of the valley (invasive and pretty poisonous too), those are my big 3 I battle with trying to contain every year.
Just google the name of the plant, invasive and your state, that will tell you more than the USDA site ever will...plunk in the plant name and poisonous and that's a quick reference if they have small kids or a lot of plant chewing pets to worry about.
Another link to pass along:
http://www.wintersown.org/How_to_Winter_Sow.htmlThey also have a facebook group which I am a member of. Can't believe I've never done this before...and I have my containers lined up in the garage for this winter already.
Some of my favorites that I'm always planting seeds for:
4'oclocks
Astilbe
Columbine (lots of different varieties)
Morning glory
Marigolds
Petunia
Pansy
Lupines
Hollyhocks
Coneflowers
There are a ton of others, basically I'll plant anything I get my hands on that won't take over, and some that will to cover certain areas.