In the early 1990s a paddler named Eric Jackson was fast enough to make the U.S. Olympic team in whitewater slalom kayak. But he lacked funds to go. So he dressed up in his full paddling outfit and brought his boat (4m long in those days) and paddle to a street corner in downtown DC. He set up a “meet an Olympian” sign and charged people cash to have their picture taken with him. Much hustle, very entrepreneurial of…
That being said, the USOC does not financially support most athletes outside of the Olympic window (meaning, from the time they depart for the host city until the time they return). This means that any activities outside of the Olympics are paid for by the athletes out-of-pocket (for most sports, this includes the Olympic Trials to qualify for the team!). Most athletes don't make money from their athletic endeavors as prize monies are usually quite small and sponsorship opportunities are usually very limited outside of the marquee sports. Generally, in the non-marquee sports (i.e., everything outside of swimming, track and field, and gymnastics), U.S. post-Olympic financial funding (i.e., salaried training) is limited to podium finishers, and many of the more niche sports further limit financial support to gold medal athletes.
[Somewhat related: The Boys in the Boat is a movie about the UW Rowing Team, which won gold at the 1936 Olympics in Berlin. At the time, athletes and teams were generally expected to help pay for the costs of attending the Olympics by fundraising (but as with fundraising today, this meant they were on the hook for any shortfalls). In the movie, Cal writes the check for the final post-fundraising shortfall of $300 so the UW team could go; in real life Cal offered a donation but it wasn't needed.]