Probably the reason you couldn't find it is that the last name
is spelled 'Costeley'.
The original poem is by Pierre de Ronsard.
https://vault2.secured-url.com/handlo/pdf/a-f/CAFM--As.pdf
How I found it:
Google search #1 - search for Guillaume Costelet. Google
corrected the spelling.
Google search #2 - search for Guillaume Costeley Cassandre.