If you're looking for perennials, Bluestone Perennials is about the best combination of strong plants and good price. They did go up quite a bit on prices this year, but so did lots of places. The plants are small, but thrive.

I would also recommend
Plant Delights at
http://www.plantdelights.com

Here is a link that might be useful: Bluestone Perennials

Have you looked at Graceful Gardens yet? When I was starting out here, I ordered from them and really liked what I got. They do send some (fairly) small plants though, so maybe not exactly what you're looking for. I do, however, still have some of the plants (and their offspring) after 4 or 5 years.

I would also recommend Niche at
http://www.nichegardens.com
They offer healthy plants at reasonable prices and nice sizes.

I like PattiOh's choices too.

I'll toss out some seed suggestions: Nigella, Cosmos, Marigolds, Morning Glories, Black Eyed Susans, Bachelor's Buttons, Flax, Blanket Flowers, Sunflowers, Blackberry Lily, Valerian, herbs like parsley/dill/fennel, Columbine, Strawberries, Agastache, Calendula.

You might also do a search through this forum. I swear it seems like we had a thread about seeds not long ago, but I don't see it offhand. We do talk a lot, so it's probably a good few pages back by now.

We have had seed swaps the past two Falls (hosted by Faltered, many thanks), so keep your eyes open since many of us have plenty to share.

Here is a link that might be useful: Graceful Gardens

I've had success with plants from Bluestone, Wayside, and White Flower Farm. The latter being the most expensive. But in late June you can usually find some bargains there.

Wayside's website is: http://www.waysidegardens.com

And White Flower Farm is: http://www.whiteflowerfarm.com

Tracy