While you can use websites such as Fiverr and Upwork, you can also try other approaches. I am not sure what business you are in particular, but there is always a way to find clients easily. Here are 2 examples:
#1 SEO
If you wanna perform SEO for clients, you can try to look for local companies in your area and check their websites. Most companies don't know what SEO stands for or what it is. Naturally, their websites are not optimized. Give them a call or email them with your offer and arrange a meetup in their/your office. Companies are always interested in getting more money. If you tell them what optimized SEO can bring them in, they want to know more about it.
#2 Web Design
Again, check companies in your area and compare their websites. Chances are you will find at least 20-30% of companies in your local area with outdated websites. Once again, get in contact with them and let them know what you can offer them and what their benefit will be. Companies always want to hear what's in it for them. That's what will ultimately lead to a positive result.
Finding Clients can be challenging in the beginning, because you will most likely face rejection, but with everything in life you need to embrace the rejection to get started. Good luck!