I need a Splunk search for finding server uptime for Windows and Linux
index= linux  sourcetype=cpu
 
					
				
		
You need to deploy a scripted input to the forwarders that calls uptime on a schedule to forward this into splunk.  It is entirely possible that the Splunk TA for *NIX has this in it.
 
					
				
		
 
		
		
		
		
		
	
			
		
		
			
					
		See if this is what you are looking for.
