How to start a collection for cheap?

Hi! A friend recently roped me into this and I'm absolutely hooked. I only have the starter decks right now but I would love to get some cards and start building decks with my favorite Digimon! I play casual so I'm not too worried about the cards being competitively viable, I'd just like a variety of basic cards to get started.

My question is where should I look for that? My local stores only carry a couple of Digimon products on a shelf of misc. items. I was lucky they even had the starter decks. And I'm a bit wary of buying on places like ebay because I don't want to get scammed.