Put a cheap for sale by owner house on craigslist and they will all answer the ad....trust me. I've had a ton of them answering my ads this week :)
You may also pick up some cash buyers while you are at it.
Yes, you can put up an ad for a house you don't have, often referred to as a ghost ad, which I am not saying you should nor shouldn't do, that's completely up to you. I'm just telling you what works for me (my ads are for my houses - not ghost ads - just to clarify :).
You can also place an ad on craigslist saying you are looking to network with other wholesalers, or an ad here on BP, make sure you are specific to your area.
A great way to find wholesalers is to call the "We Buy Houses Signs" in your target market and ask them what do they have. Most of these individuals will be wholesalers.