I registered the product and can't find my receipt

You can go back to Home Depot (assuming that's where you bought it) and get another receipt.
If you have an online Home Depot account and have a credit card tied to that account and used that card at Home Depot you can see your receipt right online and use it for the registration. Doesn't matter if you ordered online or bought it in the store, if you used whatever CC you have attached to the account, the receipt will be there.