The editor cannot find a referee to my paper after one year I submitted a manuscript to a journal one year ago. The journal is well known and has already published papers on the same subject including one of my previous papers. The paper I submitted went under review after two weeks. After eight months, I contacted the journal to inquire about the status of my manuscript, I got no response. I contacted the journal again after two months, and the managing editor informed me that the handling editor is trying to secure a referee for my paper. I sent another email three months later, the managing editor said that the handling editor is still trying to secure a referee for my paper. I asked them if I could suggest some referees and they agreed.
My question is: what should I do after this long waiting period?
As you pointed out you have several options:

Do nothing and wait for the editor to find a referee.
Withdraw and submit elsewhere.
Send the editor a list of people who might be able to review your paper.

Nobody, including the editor, can predict what will happen if you do any of these things. Doing nothing and waiting is probably the least likely to result in a good outcome quickly, because the editor's already shown they are not able to find reviewers quickly (although there's always the chance a reviewer's time frees up and they can review). Withdrawing and submitting elsewhere would mean starting over, and there's no guarantee the editor of the new journal will be able to find reviewers either. The third option is effectively a better (for you) version of the first option, but it's still not ideal, because you can't guarantee the reviewers you suggest will agree to review either. You can only tell who might be interested in your paper.
Because there's no ideal option, you will have to make your own decision. How badly do you want to publish in this journal? If the answer is &quot;very&quot;, do #3. Otherwise consider (but not necessarily take) #2.