drsfostersmith plant site is -
http://www.liveaquaria.com/product/aquarium-fish-supplies.cfm?c=768
Aquariumplants.com is another
AquaBid.com is a joke in my opinion, I've bought fish from people on there a couple of times, and in all instances the fish arrived dead. Plus people on there want ridicules prices for stuff that you can find elsewhere for half the price.
But web sites have pricey shipping cost, so I get my plants from other users on (TPT) PlantedTank.net in the for sale/trade section. Most of the time people on PlantedTank.net use USPS flat rate shipping which is between $5-$10 depending on where they live in the U.S., every person I've bought from on TPT has sold me healthy, colorful plants that were amazing for the prices. Agreed that most of the plants sold on PlantedTank.net are trimmings from peoples tanks, but it's a good way to find a great variety of plants for cheap.
And if you're looking for specific plants PlantedTank.net has a WTB/RAOK section where you can post what you're looking for!!!