No way! Are you sure? Maybe this is a recent development. There's several other Hemianthus plants, not callitrichoides, that look similar (especially glomeratus) and are reasonably common in the US. It's just that I've read so many complaints on other aquarium forums of how hard it is for people to find.
Aquariumplants.com and Aquatic Plant Central(?) Have large selections of aquatic only plants...also aquabid...can be dangerous as I am currently attempting to start a AAA group....aquabid addicts anonymous...:)
Seriously, you can get some really nice plants from aquabid...rare ones too, such as variegated baby tears, crypts and anubias (you were all correct as these all have a variegated sp...BUT...it is much more common for aquatic plants to have a reddish orange/green or reddish purple/green variegated varieties).
Oh, and it was aquariumplantscentral.com I believe(above comment)....they also sell on aquabid and are one of my favorites although user p_volitan is the best! They both have great communication, large portions, competitive pricing, and absolutely gorgeous plants!...sorry for the sales rep/ reviews moderator, but it was in answer to direct question.
Aus: you rock! Wonderful guide, well put together, thanks!
Aus, I wanted to add a plant to the list: Star Moss (Tortula Ruralis). It's a moss that some people may be tempted to plant in a betta tank because of how beautiful it is (plus, it looks aquatic), but it's not truly aquatic and will die within 2 - 3 months of being kept underwater. Some shops and eBay vendors sell it without telling their customers this.