Check local FB groups first. Should find some good bulk prices for basic building purposes. Might even find some old precons at a good price. Look up local flea markets for often old cheap bulk (can have some real funds here). As for websites, until recently, I would have suggested eBay but I can't support what they are doing in any way. I unfortunately do not know a good alternative for cheap bulk but Google might help. Also make sure to check out local shops as the ones that have been around for a little often have things like into bulk packs for free and established players will probably give you a chunk of bulk once you get to know them. Often after a FNM people will leave piles of things they don't want for new players just make sure you ask first.