This only reminds me how many more plants I feel like I ought to have, haha. I've only got, scattered among all of my tanks, several argentine swords, an amazon sword, one unidentified aponegentin sp. from a 'betta bulb', a bunch of anacharis, some riccia, and something whose species I've been unable to confirm, but which the tube said was "Bolbitis heterclita"...? It looks more like asiatica, but not quite....? Blah. Lol.
But that's all I have. I need more variety, but that can't happen until after I get a better light, I'm thinking...
Anyway, that's all another reason why I am hesitating to get the little glass cube nano (micro? ultra-nano? I feel like there should be a division for the ultra-tiny ones...)
Reaaaallly tiny tanks are sometimes called "pico" tanks--but that's usually reserved for ones that have to be filled with an eyedropper.
Easiest lighting is a lamp with a bendable or multi-jointed arm so you can position the bulb directly over the tank and easily adjust the height. Toss in a full spectrum bulb with the highest lumens you can find--done. If it can get some indirect sunlight, even better. You aren't going to be looking for rampant growth, so you won't need to blast it with high light levels.
Petco usually has both cardamine lyrata and a small variety of green leafed crypt available and I've seen dwarf hair grass, baby tears and a variety of smaller plants ("small" in either the mature form or just small because it's very young growth-either works for a micro-tank) in pre-packaged bags.
You could also try posting a WTB here--let 'em know you're looking to get a variety of stem plants and only need very short clippings. I suspect you'd be able to get a whole variety of someone's prunings (1-3" long snips rather than the usual full sized stems most people are selling) for little more than the cost of shipping--plenty to experiment with.