If you want to buy a small business, business for sale web sites are the best places to look. Bizbuysell and Bizquest are both good sources for small businesses in the US. Businessesforsale.com is good for small businesses outside the US. And DealStream is best for middle-market businesses.
If you’re looking for larger businesses (over $10 million in value), you need to use a different approach. Most large businesses aren’t listed on business for sale web sites. Instead, the bankers who sell these "unlisted" businesses create a list of potential buyers, get the seller to approve the list, and send teaser letters to the buyers. If you want to receive these teaser letters, you need to be proactive. Put together a list of intermediaries and call or write them on a regular basis (at least every few months) asking them if they have anything meeting your criteria. DealStream’s member database is a great tool for doing this.
If you have time, there’s another group of businesses that's worth pursuing: “pre-market” businesses. Pre-market businesses are businesses where owners are thinking of selling but have not yet hired a broker. Pre-market businesses are attractive because they often can be bought at a lower price. That’s because sellers do not have to pay a commission and there’s no competition from other buyers to drive up the price.
Finding pre-market businesses takes a lot of work. Start by creating a list of business owners in the industry and location you want. (DealStream’s member database is good for this.) Then, send these owners direct messages asking if they would be willing to discuss a possible sale. Most owners will say “no” or simply not respond but, if you contact enough owners, some will say “yes”. You can also create posts asking your followers to provide introductions to potential sellers. Consider offering a finder’s fee - it will produce more responses.