Its called wholesale. Look up "insert product you want to sell" wholesale on Google and you can find some wholesale sources for these items.
Wholesale still kind of sucks, since you usually can't get items cheaper than half of what they are worth retail. There's a lot of risk involved in wholesale, and a lot of work managing such huge numbers of items (the main disadvantage of wholesale is the more you buy the more you save, but unless you have employees then handling such large numbers of items is daunting.)
I suggest checking out thrift stores, yard sales, estate sales, flea markets, real life auction houses, consignment stores. Any store that is not selling items for retail can get great sources for both individual items and large numbers of items. Some warehouses has huge pallets of items that never got sold for wholesale but there is an untapped online market for them. Getting to know people are consignment stores and people who run auctions can lead you to these sources.
If you're going to get into this sort of business its going to take some real effort. You can't expect a google search to get you all the things you want to know. You've gotta get involved with people and network. Start small, and concentrate your efforts on learning the value of individual items. Pick a market, like sports memorabilia, and use eBay to learn what things are going for so you know what to keep an eye out for. Being able to judge and items actual value is not enough, knowing what it goes for on eBay is the only surefire method of knowing what you can sell that item for on eBay. You will also be getting a lot of stuff not worth anything, and if you get boxes of stuff you'll have to deal with junk you don't want. This is where having a yard sale yourself, or putting everything up for sale on craigslist can come in handy. Be prepared for a lot of organizing, and learn how to manage your risk. The fact remains, getting an item worth 50 bucks on eBay for a quarter is a lot better return then getting something for 5 bucks and trying to sell it for 10 or 15.