U-M owns most of the SPIE Conference Proceedings, but it can be a bit difficult to find where they are located.
For a proceeding published from 1990 to the present, check to see if there is a digital version in the SPIE Digital Library.
For proceedings published prior to 1990, or for which there is no electronic version, first try a words anywhere search in Mirlyn, using 'SPIE' and the volume number; e.g.:
If that doesn't work, try a title words search with the title of the Conference, but without the roman number number; e.g.:
Lasers in Dentistry
Lasers in Dentistry VI
Alternately to using MIRLYN, or if the MIRLYN tactics don't work, you can use the SPIE Proceedings Call Number Database to look up the call number for the proceeding by title words and/or volume number (but note that this database will not be able to tell you if it is checked out or not).
Failing the above, please contact a reference librarian for assistance.