These journals say that they publish expository and/or survey papers...

Bulletin of the London Mathematical Society
DOCUMENTA MATHEMATICA  
Global Journal of Pure and Applied Mathematics  
International Journal of Modern Mathematics  
INTERNATIONAL MATHEMATICAL FORUM  
Journal of Analysis and Applications  
Journal of Interdisciplinary Mathematics  
Lobachevskii Journal of Mathematics  
Logic and Analysis  
Journal of Mathematical Sciences:  Advances and Applications  
Surveys in Mathematics and its Applications  


Another possibility: *Lecture Notes in Mathematics* (Springer) or similar.  But only for longer expository pieces: Lecture Notes has a minimum of 100 pages.