How do I get a transcript for 1 course I took at a business institute that is now closed?

Was this a private for-profit education institution, or was it part of a government or other network?

If it was part of a state education system, I would think their state-level governing body would keep such records on file. If it's part of a corporation which still has operating institutions, I would call their headquarters as they have probably kept these records. If it's part of a religious educational network, again - you find out who overseas the network of schools overall and they will probably have records.

Otherwise, like if it was a sole institution which no longer exists, and which has no parent organization still in operation, I would suggest that you contact your government's office in charge of education and just try and see what you can find out. Depending on where you live there may be requirements for this situation.

As an alternative, you may not be able to get this information. Do you still have your report card or proof that you paid for the course? You might have to get the receiving institution to take mercy on you and accept the course on the basis of something other than official transcripts. If the institution is now closed, perhaps they will take pity on you and accept other records that you took the course.

First it may not even be possible,

If it was a school that was accredited, the records should be with the department of education for that state.

If not, often you can not get any records