Marketing today is basically spoon-feeding your product or information regarding your product to your target customers. Customers do not, usually, spend time looking for the best products. They choose from whatever they are hit with. So, like most salespersons, start spoon-feeding. When you don't want to spend money, use the free resources.
Write about it on platforms like medium. Medium is an open platform, which will also pay you according to the number of views your articles receive. So, you don't spend at all and you may earn, too.
If you have a small-niched product, there may exist platforms that target that particular community. You need to find such a platform and start writing about your product.
If such a platform is not available, create your own free website (free option - Google sites).
If your product is information, write about it and sell it on amazon. Amazon may actively sell your idea if you choose that option. They will charge you extra for that but it is a percentage of your sale, so you don't have to pay initially.
Market your stuff for free on social media. You can, probably, use every social medium available out there.
Design a course that teaches/ explains your idea.
User reviews and user feedbacks are two really important things. Always ask your clients for that. Place rewards for writing reviews on different platforms and feedbacks. The rewards should bring back the customer. For example, if you have a course, the rewards can be a discount on the customer's enrollment in your advanced course. The rewards bring back the customers and the reviews bring in new ones.
Hire a marketing expert and pay them in terms of a profit share.
Make short videos and share them on Youtube.
Always cite self stuff. If you are making a video for Youtube, add links to your articles on Medium and your website. Do the same on all the platforms you have your presence on.