Add following code under the Product form where you want to display Buy now button

<div data-shopify="payment-button" class="shopify-payment-button" >
<a href="/cart/{{product.selected_or_first_available_variant.id}}:1" class="btn shopify-payment-button__button " tabindex="0">
<span id="AddToCartText">Buy Now</span>
</a>
</div>

add styling according to your store design

Example store: 

Link: https://bediuzzaman.myshopify.com/

Screenshots: https://nimb.ws/9f3oic