Most pet stores sell ADFs in my experience. They can be tough to see so just ask where they have the African Dwarf Frogs (sometimes called Aquatic Dwarf frogs).
My first time looking for them I walked in and scanned the tanks 4 times and didn't see any. Didn't even see a label for them. I asked the associate if they carried Aquatic Dwarf Frogs and he walks over to a tank, lifts up a huge decoration and there were about 5 sitting there under it.
Now I just walk straight up to the associate and ask if I don't already know.
As for shopping online, I do it occasionally. iTunes and the like mainly. I just try to limit my transactions and places that have my info. I also have encrypted wireless, firewalls etc but those don't protect the recieving end, protected or not.
I also like the idea of supporting the mom and pop shops and the real people. Same reason I refuse to use the self check-outs at the supermarket. I don't care if it takes longer or is harder to do, I prefer interacting with people.
And now I sound like some 90 year old technophobe... I swear I'm not!
I really do like the frogbit though. I also like the idea of a flowering plant in my tank. My main concern is the leaves. They may block too much of the light to the rest of my plants or take over the top and make it hard for my Bettas to get air. I'm probably over estimating the size but I worry.
Let me know how yours do, It'll probably be a few weeks before I get around to actively shopping for something.