Anacharis or Cabomba?![]()
Maybe Wisteria? (Hygrophila difformis)
Hygros in general are easy and common.
najas grass (Najas quadalupensis) aka guppy grass is happy to float OR be rooted. I have some rooted in sand in a 10G with a 15W t8 lamp and it's doing great.
Plus - it doesn't have that annoying hornwort tendency to shed it's leaves when it's mad at your light or your water.
Finding it at a local store? I don't remember ever seeing it at a chain store. Ask around at a local fish club...someone may GIVE you a bag of it. I certainly would..."got plenty"! You may find some in the For Sale section here on AC too.
Anacharis aka Elodea would be my 2nd choice...but it's a but finicky to get it to root and stay rooted IME. Works OK as a bunch plant tho.
Cabomba's not really an easy plant...it prefers more light and care than hornwort.
The OP was not looking for an "easy plant". He/she was looking for a plant that did not float around and was easy enough to find in his/her local stores.
If the OP has enough light to grow Amazon Swords; he/she has enough light to grow cabomba in my humble opinion.![]()
Well then that's where our opinions differ.