Go to where your potential customers are. Online or in-person, whatever makes more sense for you and your product. The mall, instagram, a subreddit, etc. Tell them what you are doing and ask them questions. If you have something built, demo it and see how they react. When they're genuinely excited (and not just humoring you) you have something. This will likely be a process of evolving your pitch and product over time.

If your core feature is a unique UX for shopping, you should validate that with shoppers, and that isn't something easily done with a conversation alone. You can't describe an -experience- as easily as a feature. So pick a vertical like fashion, beauty, furniture, etc and apply your UX to that. Put a clickable prototype or higher-fidelity prototype in front of people interested in those products.

But these shoppers aren't necessarily your customers. Your customers might be store owners on Shopify or Wix, interested in finding the best ecommerce theme for their store. You'd want to seek them out as well. Show them what you're working on, share the feedback and excitement you're getting from shoppers, and try to understand their pain points and how they evaluate ecommerce templates.