Cardinal tetras and Harlequin rasboras would look awesome in big schools!
You ask for online places to buy fish and plants. The places I've bought from and recommend are:
Rachel O'leary from Invertebrates from Msjinkzd
She carries nano fish, snails and dwarf shrimp!
Current Stock list - Invertebrates by Msjinkzd
I bought Cardinal tetras and panda corydoras from them and also aquarium supplies from their other website Dr's Foster and Smith.
For live aquarium plants I recommend fully, PlantedaquariumsCentral.com
Planted Aquariums Central Live Aquarium Plants
I've ordered from her twice and both times receive beautiful, healthy, well packaged plants! Both times I received extra plants
If you want I can PM you the link of the inbox the link to unboxing video I made of the first plant order I made. You have to see the beautiful Anubias Nana she sent me!