I haven't had to buy any plants in a really long time-The only mail order plants I bought was from sweetaquatic-but that was a really long time ago and so I can't say much about them. One good place to get plants is from your local aquarium club-I would google search your state/country and see if you have a local club or look in the back of most aquarium magazine for a list.
Good floaters-water lettuce is one of my favorites-but you can float any specie of plant if needed-especially if it is short term. The stem plants are better to float compared to the rosette type plants-Then you have the rhizome plants like ferns, anubias that will do well floating too-even long term. I float a lot of different species of plant to get them started rooting. Since this can sometimes give them a head start to a degree due to them being able to obtain higher CO2 from the atmosphere instead of depending on the CO2 content of the water.