How do I find my original purchase date of my iPad?

Your original sales invoice will provide the purchase date. If you have lost your invoice, you may be able to estimate when you acquired your iPad by checking your warranty and support status - this being based upon initial activation of your iPad:

Check Your Service and Support Coverage - Apple Support

Outside of your receipt / invoice, there's no where else to find that or its original price.

Well Go To Your Store Provider Or The Website You Bought It On, And Look For It In Order Again.