Measure the opening and tell a building supply place what you want, and they should (probably) be able to order it for you. Won't be as cheap as the off-the-shelf stuff, but that's the way it is.
Edited to gripe about trim: To me, the extra cost of ordering the right size is far, far, far better than the cost/time/headache of filling/trimming the opening. I hate doing that.
Last edited by Pine_Tree; 04/12/14. Reason: added second paragraph
