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.
