There are many great places to stay near Cooper's Lake.  You'll find a variety of hotels, bed & breakfasts and campgrounds at these helpful links:

https://www.visitbutlercounty.com/accommodations

http://visitlawrencecounty.com/local-attractions/stay-over/

http://www.visitmercercountypa.com/Accommodations-In-Mercer-County-PA.htm