The Prospect recently wrote about how 
Joe Biden's foreign policy team got rich, and it just so happens that most of the people mentioned in the piece are current or former think tankers.
Here are some excerpts:
They had been public servants their whole careers. But 
when Hillary Clinton lost the 2016 election, two departing Obama 
officials were anxious for work. Trump’s win had caught them by 
surprise. 
Sergio Aguirre and Nitin Chadda had reached the most elite quarters 
of U.S. foreign policy. Aguirre had started out of school as a fellow in
 the White House and a decade later had become chief of staff to U.N. 
Ambassador Samantha Power. Chadda, who joined the Pentagon out of 
college as a speechwriter, had become a key adviser to Secretary of 
Defense Ash Carter in even less time. Now, Chadda had a long-shot idea.
They turned to an industry of power-brokering little known outside the 
capital: strategic consultancies.  Michèle Flournoy had served as undersecretary of defense for policy from
 2009 to 2012. Both Aguirre and Chadda had known her well in the Obama 
administration. Since leaving office, she’d spent several years in 
consulting and was hitting her stride. With Flournoy as senior adviser, 
Boston Consulting Group’s defense contracts
 grew from $1.6 million in 2013 to $32 million in 2016. Before she 
joined, according to public records, BCG had not signed any contracts 
with the Defense Department.
Flournoy, while consulting, joining corporate boards, and serving as a 
senior fellow at Harvard’s Belfer Center, had also become CEO of the 
Center for a New American Security in 2014. The think tank had $48 
million on hand, and defense contractors donated at least $3.8 million 
while she was CEO. By 2017, she was making $452,000 a year.
Others mentioned in the piece include Tony Blinken (former senior fellow at CSIS), Nicholas Burns (former visiting scholar at Wilson Center and on board of directors of Atlantic Council and CFR), Kurt Campbell (co-founder of CNAS and former scholar at CSIS), Tom Donilon (distinguished fellow at CFR), Wendy Sherman (on board of Atlantic Council), Julianne Smith (adjunct senior fellow at CNAS and formerly at CSIS and German Marshall Fund), Jake Sullivan (nonresident senior fellow at Carnegie Endowment for International Peace), Robert Work (former CEO of CNAS), Dan Shapiro (visiting fellow at Institute for National Security Studies), and Avril Haines (nonresident senior fellow at Brookings and on CNAS board of directors).
The article notes that Flournoy went on to form WestExec Advisors with Tony Blinken, and they partnered with Jigsaw, which is Google's in-house think tank.
Here is an Al-Monitor piece on some of Biden's foreign policy advisors.  Besides the above-mentioned Tony Blinken, Jake Sullivan, Julianne Smith, and Nicholas Burns, others advising Biden include Colin Kahl (former fellow at CFR and CNAS), Brian McKeon (on leave as Senior Director at Penn Biden Center), Jeffrety Prescott (Senior Fellow at Penn Biden Center), Ely Ratner (CNAS), and Elizabeth Rosenberg (CNAS).
It notes that many of Biden's foreign policy advisers come from four entities: Foreign Policy for America, National Security Action, Penn Biden Center for Diplomacy and Global Engagement, and WestExec Advisers.