The following gives a method that will work if you've ever backed up the iPad to a computer (and another idea or two): iOS: How to find the serial number, IMEI, MEID, CDN, and ICCID number
Also if you updated your support profile it will be there: https://supportprofile.apple.com/
It really doesn't matter whether you have access to iCloud because your serial number wouldn't be there anyway. My response to the original poster gave the possible locations. If you like you can log into iCloud at https://www.icloud.com using your Apple ID.
If the iPad was running iOS 7, the thief/finder will not ever be able to use it.
iCloud: Find My iPhone Activation Lock in iOS 7
Apple (and no one else) can not assist (with serial number or iCloud) in finding a lost or stolen iPad.
Report to police along with serial number. Change all your passwords.
These links may be helpful.
How to Track and Report Stolen iPad
Reporting a lost or stolen Apple product
What to do if your iOS device is lost or stolen
How to Find a Missing iPad with FindMyiPad
iCloud: Locate your device on a map
iCloud: Lost Mode - Lock and Trace
iCloud: Remotely Erase your device
Report Stolen iPad Tips and iPad Theft Prevention
General steps to follow for a Stolen iPad
How to recover a lost or stolen iPad
How to Find a Stolen iPad
What NOT to do if your iPhone or iPad is lost or stolen
Apple Product Lost or Stolen
Oops! iForgot My New iPad On the Plane; Now What?
If you don't know your lost/stolen iPad's serial number, use the instructions below. The S/N is also on the iPad's box.
How to Find Your iPad Serial Number
iOS: How to find the serial number, IMEI, MEID, CDN, and ICCID number
Check the serial number (located on back/bottom) in this lost/stolen iPad database. However, most owners are not aware of this database and don’t enter the serial number.
Stolen iPad Database
Also do a search, using the serial number, on your local Craig’s List. Some users list the serial number of a stolen iPad to thwart the sale of a stolen device.