If you're serious, start networking. You probably know somebody who does import/export, wholesaling, manufacture, etc. Start asking about getting things for wholesale cost. You basically buy a case, carload, pallet etc of whatever and then sell it for a profit.
Also, see if you know someone who is in fulfillment. They are the people who take things from order stage to sending out stage. They know more than anyone about what sells and where to get large quantities of anything at wholesale prices.
Other standard methods of finding product are Police sales, Government auctions, business auctions (the back page of the NY Times sunday classified is classic), bankruptcy sales.
And the other standard advice is "find out where your dollar store buys their merchandise."
Good luck.