how can i contact the original owner of an ipad pro that i bought

I presume that you are faced with an Activation Lock...

Here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be turned off:

https://support.apple.com/en-us/HT201365

https://support.apple.com/en-gb/HT201441

As you will discover, unless you are the owner of the AppleID that was last used to activate the device, or unless you can independently secure the cooperation of the previous owner, you’ll not be able to defeat or bypass the Activation Lock. Without documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple won’t assist - and nobody here will be able to help you. Similarly, Apple will never assist with tracing owners.

If you are the owner of the AppleID that previously activated the device - and you have forgotten your AppleID or associated password, these can be recovered here:

https://iforgot.apple.com

To recover your credentials, you’ll need access to one of the following:

• Your primary email address mailbox that corresponds with your AppleID
• Any of the secondary/recovery email addresses that should be configured for your AppleID account
• Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with your AppleID

Unless you AppleID account has not been fully/correctly configured, or has been seriously neglected, recovery of the AppleID and password should not be difficult.

More information about recovery of your AppleID:

https://support.apple.com/en-us/HT201354

More information about recovery of your AppleID password:

https://support.apple.com/en-us/HT201487

When you recover access to your credentials, you would be well advised to log-in to your AppleID account from a web browser - and verify/update any email addresses and trusted telephone numbers:

https://appleid.apple.com

Contact the person you bought it from. Are they not the original owner?

There is no list or iPad owners if that's what you are thinking. That would be a violation of privacy and even worse to just hand out personal details of people to anyone who asks simply becasue they owned an iPad.

Agree with Phil. Isn't whoever you purchased the iPad from the original owner? I'm guessing you have purchased an Activation Locked iPad, and you are hoping to get help from the original owner. Simply said if this is the case and whoever sold the iPad to you isn't responding or doesn't know the Apple ID or Password, you now own a useless device that Apple won't help you with and we can't help you with.

Activation Lock can only be turned off by the person who put the lock on the device.

See this link about buying used devices --> The All Too Common SAD Reality of Buying … - Apple Community