The simplest solution is often best: set up an online suggestion box. By which I mean you should ask clients for feedback after purchases — especially regulars — and make your contact page easy to find. If there is something people crave to see in your store, they won’t be shy about letting you know.
That said, people often don’t know what they want until they have seen it, so you need to be creative when looking for new products. So check what other stores in your niche are stocking, browse the new arrivals sections at whatever retailers or suppliers you use to get your goods, keep an eye out for products that are trending on social media, etc. Remember: not every new product needs to be announced with a ton of fanfare. You can just order a small run of items you find interesting and quietly put them up on the site to see if anyone shows interest.
Of course, working alongside a data consulting agency like https://metriclabs.com.au/data-analytics-consulting/ can also be helpful when trying to keep track of how well different products are performing.