It takes a while for those unusual sized fans to get out into the retail channels. Even now, the 250mm fans aren't that widely available except in a case. But you can readily get the Antec 200. About the only thing in between that and the 120mm are the few 140mm out there (Aerocool, Yate loon). I guess thermalfx.com has some "172" mm fans. And you could check the Mouser.com online catalog - they are full line stockists but you'll pay premium prices for small quantities. And sometimes bgmicro.com has some odd sizes, maybe allelectronics.com too.
.bh.