If you do go for online retailers like Ebay or Amazon, beware of bait and switches.
As in they advertise something but you get a false product like for example, broken batteries.
This shit happened to me once when I was looking for battery replacements for Pokemon stuff like old cartridges and the Pokewalker
If possible, go to a physical brick and mortar store.
If not, I would recommend one of those specialized online game stores like any flashcard sites you actually trust.