Was looking around on digikey and found this: http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=217896006&uq=636214902378501393
Seems like a nice selection and all look sized right for N-scale. I'll probably get it to experiment with. The MRR industry sold speakers I've been trying just aren't cutting it.
That link didn't take me to any specific speaker . . .
Digikey sells Soberton speakers in 8x12, 9x16, 11x16 and 18x13mm sizes. Some of the sizes come with an adhesive "tape" or "foam" on them - I'd skip this and just get a bare speaker. I've used the 8x12 in the Keystone Details 3D printed enclosure, and found the sound excellent (for the size of the speaker; remember that it isn't going to sound like a pair of Altec Lansing Voice of the Theater horns with 15" woofers). Digikey also sells 9x16 and 18x13 speakers from CUI, but I think Mouser has a better selection of the CUI speakers. I've never used the CUI speakers, nor any of the Soberton speakers other than the 8x12, but the specs on all these look reasonably similar to the Knowles speakers that I still have; I expect all these would be fine in a properly-sized sealed enclosure (at least 500 cubic mm of free air space). It really doesn't matter how you get to the 500 cubic mm of air space: the enclosure can be a deep box wrapped around the speaker; it can be thin but longer or wider, with the speaker offset to one side (like what I've done for my SD40-2); it could be a triangle, a parallelogram, a sphere, whatever. The shape doesn't matter. What matters are (1) having the speaker sealed in the enclosure (no air leaks) and (2) that you get to that 500 cubic mm of air space.
John C.