AFAIK one of the Big Four usually provide the major pro franchises with tax prep and auditing services, but they also have their own accounting staffs.
It hasn't been properly updated in awhile, but according to the link below the Cavs recently had two staff accountants, a senior accountant, an Accounts Payable Coordinator, a Director of Accounting, a financial analyst, a Director of Finance and Development, and a CFO.
https://basketball.realgm.com/nba/teams/Cleveland-Cavaliers/5/staff-members
It's an area I would love to work in myself. I've tried using my experience in the sports media to stand out from other applicants, but without success. The “cool factor” obviously makes them desirable jobs. Based on my research, the two most common routes seem to be Big Four auditor with experience working with pro sports teams, and working in industry for one of the team's business partners, such as the stadium or concessions operator.
As for individual athletes, I know there are accountants that specialize in tax prep and planning for them. You are most likely to find them in the areas where players live, which are often wealthy communities near their practice facilities.
Here's an example of a firm with offices in south Florida:
https://dbllp.com/accounting-for-athletes-entertainers/
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