ipad was purchased from third party but still lock on find my phone.

Without assistance from the previous owner, the iPad is useless to you.

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

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Turn off Activation Lock – Apple Support

As you will discover, unless you are the owner of the AppleID that was last used to activate the device, you’ll not be able to defeat or bypass the Activation Lock. With documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple may be prepared to assist - otherwise they and nobody here will be able to help you.

The All Too Common SAD Reality of Buying … - Apple Community

If the seller cannot or will not help you remove the AppleID used with Find My, then you have a useless device. You cannot remove it yourself.

How to remove Activation Lock - Apple Support