Lived in a common law relationship for over 20 years. The man left me threadbare and incapacitated financially. How can I receive emergency spousal support?

Do you have children together living at home? What state are you in if indeed you are in the USA? Do you work, or have joint property together?

Do you have any joint accounts? What area are you in? Are there minor children involved? Spousal support and child support are ordered by the court. That being said, there may be some assistance available in your area until you have an order from the court in your area.

First, any question on law needs to include your general locale as laws vary by area.

Most areas no longer recognize common law marriages. So your failure to legalize your relationship will probably leave you with nothing.

"Common-law marriage in the United States can still be contracted in nine states (Alabama, Colorado, Kansas, Rhode Island, South Carolina, Iowa, Montana, Utah and Texas) and the District of Columbia. ..." Common-law marriage in the United States - Wikipedia, the free encyclopedia

So, if OP's 20-year relationship didn't happen in one of those jurisdictions(or another US jurisdiction during a time when common law marrige was recognized in that jurisdiction), assuming it was in the US , it isn't a marriage.