There are actually several species of Hydrocotyle, and I wouldn't call any of them low light exactly. Some are more easier than others. In their natural habitat they all grow as a bog plant in only inches of water, usually with their leaves above water. Hydrocotyle leucocephala is perhaps the easiest to grow submersed because it adapts to underwater growth and grows faster underwater than the other species. Hydrocotyle vulgaris is more sought after as a foreground plant because it tends to creep horizontal to the substrate with single stem leaves growing straight up looking like little mushrooms or "umbrellas".
As a floating plant, Hydrocotyle leucocephala grows much faster. In traditional Dutch aquariums they use the plant growing on the surface in the rear of the aquarium to keep nitrate and ammonium levels low, and in the aquascape in the front corners growing from the substrate up to the water surface arching over to create a natural frame to the aquascape.
in the photo below, you can see the pennywort growing in the right front corner as a "frame" in a traditional Dutch planted aquarium
Brazilian pennywort shouldn't really be that hard to find. Its grown commercially in Florida, and several of the big online sellers like aquariumplants.com and azgardens.com sell it for less than 2 dollars for a whole bunch.
Cardamine is from Japan mountain lakes and likes cold water and lots of light. It grows slowly;
