One of the best ways you can find foreclosures in the Darke County area (or for that matter, anywhere in the US) is to use RealtyTrac - as of today, there appear to be 167 properties that are either distressed or bank owned. Without having to pay for a subscription - if you know that there is a property on Warren St in Greenville, you can drive Warren St and look for the yellow tag on the door or look for high grass or other visible signs that the property has been abandoned. Then, go to the courthouse (or look it up on the Internet - key words in Google are Darke County Ohio Property - then, click on the link to the online Auditor) There, you can find the owner of the property and a bunch of information about the property.
Finally, if you don't know what you are doing... use a real estate agent to assist you. Try to find a real estate agent that specializes in these types of transactions.