I'm horrible at growing Java Moss (just doesn't like my water I guess) so I haven't really paid moss much attn. Fissidens may grow locally. I don't think it's hard to find either. The attached pic was taken on the VA side of the Potomac a couple miles north of Mount Vernon. Some of that looks like Fissidens. That's all growing on soil IIRC.
I got a huge wad at the January GWAPA meeting (really great group for those of you whom haven't checked in yet). Just used fine black cotton sewing thread to bind it to driftwood, will eventually rot but by the time it does everything will have rooted/attached.
I thought it could grow either way, emergent as long as it stayed moist. It can definitely be found in America and "sometimes emergent due to fluctuating water levels." The photo was on a slope several feet above the waterline so maybe it's not fissidens after all.